I'm sorry you struggle. Others have already shared good thoughts. I'll just add some of mine in case it helps you any.
If you worry about his penis being bigger, remember, you can shop. Be a three penis guy, if you want, with whatever nature blessed you with and whatever you want to put in 2 thigh harnesses. Remember you have a tongue and fingers. Whatever sex toys you and partner want to play with, you can do that.
Sharing sex goes
beyond "penis going into mouth, vaginal or anal openings." Shared pleasure can happen in a lot of ways. And note I say "share sex" and "shared pleasure," and not "get laid" or "orgasm."
I'm just NOT going to orgasm at certain times of the month. That's just what it is for me. I can masturbate with all kinds of things, share sex, and nope, no orgasm. That doesn't mean I can't have a good time. It just means my orgasm is not a goal. So some dude just humping and pumping, trying to get me to orgasm, even though I say it's not gonna happen? Just so HE can have an ego boost from trying to get me to ANYWAY, to prove his studliness to himself? Total turn off to me, because it is not shared pleasure then, is it? He's not even listening. I don't care how big his penis is, if he's here treating me like a sex toaster that's just dispensing sex and orgasms for his jollies. I'm not a machine.
What kind of sex sharer are you? Do you think about it as "shared sex" and how you participate as a lover?
If you worry about a better, fitter body-- fit for
what? We don't all have to be like a professional athlete. But if you want to get fitter for rollerskating, hiking, gardening, whatever activities you and partner enjoy, you can do that.
You request an agreement that you two will stick to sharing about sex-health info basics, like safer sex practices and labs, and NOT overshare details like that. Other partners may not have consented to that, so it would be a violation of their privacy. She also should not blab too much information about what you look like naked, or what sex is like in the (you + her) dyad to other people, without your consent to tell and their consent to hear. It is not only her info; it belongs to you too; and the ears belong to the receiver. Just because you are okay with her telling doesn't mean they are with okay hearing it. The same goes for you. Just because other people are okay with her telling doesn't mean your ears consent to hear.
You do not blab TMI about her to other people you date without her consent to tell and their consent to hear.
Over here in THIS dyad, you have opportunity to learn more about sex ed in general and talk to partner and ask what pleases her at THIS chapter of life. I will tell you right now, the stuff I liked in my teens and 20s is way different than what I wanted when I was pregnant, and now, in perimenopause. You might reflect on how your preferences have changed over time, too.
Mags touched on how the patriarchy can hurt people, including men. I find it annoying how many people buy into the idea that "men are only good for their dick and their wallet."
The human brain is also sometimes an odd place. Sometimes it can be like, "I don't even have female equipment. It's not like I could compete there," so you are able to
relax a bit more at the idea of her sharing sex with women, because it's apples to oranges. But you do worry about men, because you DO have male equipment and you're worried she'll be comparing apples to apples.
Other times it goes the other way for guys, like, "OMG! I don't even have female equipment! It's not like I could even compete! Now what?!" And they worry about the idea of their female partner sharing sex with women, because it's apples to oranges. But they feel more okay with men because they DO know that arena, and feel all right there, and think their own apples are okay. But they have no oranges. Ack!
What are you insecure about? You might read "Polysecure" and "Polywise" by Jessica Fern. There are other books, podcasts, websites and other resources. Take your time reading and listening.
Is "polyamory" even the type of non-monogamy you want? It's okay to contemplate polyamory and at the end decide "Okay. I thought about it long and hard. But no. I think it's fine for other people, but it is not for me."
You are not obligated to do it.
If you think you could benefit from talking a a poly counselor while you think all this out, you might try seeking one at:
Polyamory-Friendly Professionals Directory
www.polyfriendly.org
HTH!
Galagirl