My husband is having a really hard time being comfortable with me being intimate with my boyfriend.
The short answer is that he needs to re-frame how he is looking at this situation.
Instead of allowing these doom images to continue roaming around in his head, he needs to re-frame it into something closer to gratitude. He needs to start focusing on how glad he is that you are able to have so much love in your life, that he wants good things for you, and that he needs to learn to embrace that your freedom and happiness is connected to his freedom and happiness.
He's trying very hard to do what he can but I'm just looking for advice to help him. He keeps saying he can't stop picturing me and my boyfriend together
The long answer is a bit more in-depth, and has more to do with how you respond, then what he is doing.
Insecurity is just what happens when we try to imagine our future, and the only predictions we can make are scary. It doesn't mean that the future is actually scary, it just means that it's all we can picture (or at least the dominant picture).
When I start a new job, my stomach is in knots and I dred every moment because I am certain that I'm going to screw everything up and make myself look like a complete idiot. While history doesn't support this assumption, it tends to be something that I carry with me and need to work through with each time I get a new job. The solution is to go to work, be nice, pay attention, and learn the new tasks and dynamics. Once I've established an actual pattern (as opposed to an imagined pattern) I can start to relax and let go of this insecurity.
The mistake that a lot of people make is to try and avoid the source of the insecurity and confuse ourselves into thinking that is a solution for the insecurity. As with anything else, we only gain strength and competency by being tested, working through it, and learning from our successes and failures. So when it comes to dealing with "insecurity about my partner rubbing their genitals against someone else's genitals", the only way to actually grow past that is to establish a new pattern of viewing it, which is only going to happen by working through the situation.
I recommend you continue living your life, being kind and supportive of his feelings where you can, draw and maintain healthy boundaries, let him be where he is without taking his issues on as your issues, be honest and don't coddle.
and it actually affected his proformance with me last night. Any advice is welcome please
"Male performance" is a big issue with me. My issue isn't whether or not men get erections, but the fact that we hold that as such a big deal. This is a social tradition that I want to encourage everyone to step away from. It is corrosive to our ability to be sexually expressive with each other because it is a strict gender norm that insists there is only one way to be a man.
Lesbians have perfectly healthy and rewarding sex lives and neither of them manage to get a blood engorged erection that they slip into one another. Sex is more involved that an erection, and some of the best sex I've ever had didn't involve one. One of the most rewarding sexual experiences I have with my partner is to have a shower and scrub each other from head to toe; I leave the shower feeling an almost euphoric sense of connection and it is absolutely beautiful.
I want to encourage you (and everyone) to look past this one rather inconsequential biological response, and instead learn to be present, expressive, intimate, curious, experimental, and enjoy the moment. Stop pretending that sex can only happen when there is an engorged penis present, because that couldn't be further from the truth.