I don’t know how much help I can be, because outside of fanfic, relationship advice is completely out of my wheelhouse. I’ll try though.
First… obviously I don’t know the first thing about your friend, but if he’s uncomfortable around you just because you’re not dating, or because you’re not comfortable having sex, or whatever it is you’re doing that doesn’t fit in his little circle of “acceptable sexuality”, then as far as I’m concerned he needs a reality check and a few infographics about asexuality shoved under his door.
Personally, I don’t understand people that think asexuality is something strange. With the whole history behind homophobia, I’m not saying I condone it, not at all, but I get why people are homophobic. They’re told being gay is bad and they start to believe it. But barely anyone’s heard of asexuality, so any prejudice against it comes from where??? As far as I can see, it’s just people being uncomfortable with people being different from them. If your friend is that type of person then, well… unless you’re willing to try and change the way he reacts to things, then you’re probably in for a long and tiring ride.
Does your friend know about the bisexual part of your sexuality? Or is this all about the asexuality? Because if he doesn’t know, then the first thing that needs to happen is that you gotta tell him what’s up. I just really hope that’s a safe thing for you to do. I’d hate see a friendship lost because of communication failures or misinformation.
Mainly, I think your friend needs things explained to him. It’s probably just lack of information and understanding making him uncomfortable. Maybe enlist the help of his girlfriend if you’re on good terms with her. Explain it to her first and get her to understand first. Then take it to the other guy. Maybe after you’ve talked to the girlfriend - and hopefully she’s supportive - have a sit-down dinner with the three of you. Make them both comfortable, then tell them what you need the guy to understand.
Also, please never forget: the problem is with your friend, not you. You don’t need to do anything differently to make him accept you. You can explain as much as you like, but if he still thinks you’re doing something wrong, then I think you’d be better off without him. It sucks, but if all he’s doing is making you feel bad for something you can’t change, then he’s not worth it.
Good luck friend <3 (Let me know if there’s anything else that comes to mind, but again, I’m not sure how much help I can be…)