My past as a serial monogamist does make me wonder if I'd be happier in an open or poly situation.
That is issue number one. We all know open model relationships are more common in gay male culture than in MF or FF ones. But you need to take the time to see which better suits you personally. Maybe you're polysexual but can only love one at a time. Maybe you can find "sacred sex" with several partners. I can and do feel the divine when I am with any and all of my lovers. It may not feel sacred to them, but it does to me! But if feeling "sacred" for you means monogamy, and you don't want sex with several partners, that is your choice.
However, I'm having a hard time dealing with the idea of him sleeping with someone else.
I'm insecure- the man he slept with has a gym body, he's older, has money and is more masculine than me.
My friend/ex assures me that is just a matter of variety... I feel inferior and like the only reason he wants this is so that he can be with men who are more muscular than me.
OK, your vanity and pride are impacting your sex and love life. Perhaps you could let go of some of the vanity. Perhaps you exercise more if you feel your partner is disappointed in your body. I know that some newly poly people have "let themselves go" in comfortable monogamy, and do start back into more care about exercise and diet when their partner begins to date. Not just to look better for their partner, but also because since the partner is out more, they have more free time and decide to work off some of their stress at the gym, or in sport.
I wonder if the other man is better in bed... what positions they got into and if they did things we haven't done or haven't done in a long time... if he's giving this person who he just met more than he gives me.
You know what? Some of my partners ARE better in bed than others. But all of them are damn good in bed (couch, blanket in front of the fireplace, kitchen counter, shower, tent, back of the van, etc). If they weren't, I wouldn't be dating them.

My LTR gf might not be as objectively "good" in bed as my aggressive sexy male Dom, but she is more tender and cuddly and kissyface and praises me more, and showers with me, and loves the same TV shows I do, and loves my cooking and decorating skills. The Dom doesn't even know what my favorite TV show or meal is! He might be colorblind for all I know, and unable to appreciate the fall display of collectibles I have on the mantelpiece right now. So, just because miss pixi may not make me cum as hard, or as often, does not mean I value her less. When you seek variety as a polyamorous or polysexual person, you don't look for the "best" person (whatever that means), you get off on appreciating each person's *uniqueness.*
I'm disappointed in myself . I feel like I let him down- like I wasn't good enough or adventurous enough...
Had your sex life actually gotten stale and infrequent, or is this just a fear?
I am angry that I don't know how to just be ok. I don't know why I have to make sex such a powerful thing. I wish I knew how to just let it be something free. So many of my friends are able to do so...
The more open model in male gay culture is more common, since all men are trained to believe frequent casual sex makes them a "stud." (Of course, women are taught that having frequent casual sex makes them a filthy tramp.) But your preference as a human being is quite common. Many people do honestly prefer being committed to just one other, and it brings them joy. Being polysexual makes them anxious. Try not to judge yourself as you experiement with open models.
but the thought of him sleeping with someone else... feels like he's stabbing me in the heart and saying " I don't care if this hurts you".
And your feeling is valid. He may not be thinking that, and probably isn't, but the fact remains, you ARE hurt. If he needs to be poly though, the only way to make the hurt go away is to leave him. It's not fair, and doesn't work, to demand he be mono.
Some couples, where either or both of them are poly, can choose to "close" for a time, or permanently, but it's a choice each of them has to make for themselves individually, for personal benefit and not just to please one while the other feels stifled and suffers.
Fear of loss there is some of that but... We seem to have a very hard time staying away from each other. I do believe that if he thought I could be a partner who doesn't chain him down and make him feel "owned" then I'm sure he'd want to be back together. However, if I am polyamorous and become comfortable with it, I'm not sure he'd be my primary partner. In addition to sex he wants a lot of freedom... I enjoy having someone I can count on to make me number one... I may want to seek a more relationship minded primary partner.
Good insight.
Experience He's so much more open and free with sex than I am. I know he will have so many experiences I'd never consider... He will probably also have more sexual partners than me. He... will be with men and women. He's also less discriminating than me and will sleep with people I find unattractive...
Maybe it squicks you out to be with someone who is so undiscriminating compared to you.
He's so skilled in bed. He's the first person to really open me to new things sexually and it meant something to me. He's the first and only man I have bottomed for... He's so free about giving it to others it makes me feel like I'm not special. For years I let myself believe that I was privileged that he gave that part of himself to me.
I am a little confused whether you two switched up who got the anal, and what he is doing now, but either way, I sense your insecurity about who he's fucking now, whether as the penetrator or penetratee.
Jealousy Will this man take his time away from me?
It could. You need to assess how much time you need with your guy... Talk it out. Maybe you could still have as much time, and he would drop a hobby or something else time consuming, to be with another partner. Otoh, maybe you'd grow to enjoy more "me time."
Will it change our sex life? I wanted to have sex the other night and he wasn't up for it. I immediately felt heartbroken because he'd done it with someone else 2 days ago and now I felt like he was holding out on me or exhausted himself with someone else.
Could be, or he could've just not been in the mood. Had he ever not been in the mood before?
He is online right now and I keep thinking that he's planning his next tryst with this man. I keep wondering how I'm going to suffer in comparison.
Comparison is a bitch. Generally there is less jealousy when you trust and really understand what your partner is getting out of their other relationship(s). I've been on both sides. I wasn't (and am not) jealous of my gf's others, but I was of my bf's since I didn't know or trust him as much, and I also questioned his taste in partners, and his lack of ethics in how he dated.
Disgusted by his choices . He's chatting with a girl I've met and find to be unattractive, he's hooked up with another guy who I can't understand how anyone could want to see unclothed (and who claims to be straight). Now he's slept with a man who is muscle bound and old enough to be his father...
These concerns can definitely affect your desire for your partner! Sometimes you can grow to accept it, and sometimes you can't. With my ex bf, it affected my desire for him to such a huge extent, it was a large part of the cause of our breakup. It wasn't their physical attractiveness, or lack thereof, as much as it was their personalities, their ethics, and the fact that 3 women in a row he chose to date had herpes. Herpes isn't the end of the world, but both he and I are free of it, and I just really didn't want to risk getting the disease. Also, we had been fluid bonded, and I was not really thrilled to consider going back to condoms (not that condoms entirely prevent herpes anyway).
My friend/ex is a small guy who looks very young and weighs about 140 pounds. I am older than him and I'm a more masculine build but I'm not this man. The thought of this man, who has three children and just separated from his wife of 17 years being inside the person I love makes me sick to my stomach.
This is the messy ethics idea. It can give one pause, for sure.