I've had a friend since highschool who is a male, we're very close, we go hunting, fishing, 4x4ing, camping.... a lot of things. While my husband and I were dating he didn't think anything was wrong with it. BUT, had it have been a problem I would have made extra efforts to make HIM comfortable with the situation. That's part of being in a relationship. It's called compromise.
I don't think it's right of your boyfriend to go to a place you're not comfortable with and leave you at home pregnant and emotional. Just not fair.
I too am pregnant right now and I'm SOOO thankful that my husband understands my somewhat selfish demands are tempororary and usually driven by hormones. Perhaps that comes with age, but, he wouldn't dare leave me at home right now so he could go out drinking. It's not just ME that's pregnant. We BOTH are.
I think you need to sit down with your boyfriend and explain that right now, being that you are emotional, insecure, and needy (which is OK, you're pregnant), that you need him to be there for you. Do this while you're in one of your good states. These conversations are never best started while emotional. Trust me on that one. LOL. Make him aware that he needs to be aware of your feelings a little more, and that sometimes it might seem crazy to him that you're asking him not to go out with a friend, but by doing so he is supporting you.
Having said that, being pregnant isn't an excuse to be a crazy neurotic demanding person. I totally understand where you are coming from, but at the same time, asking him to change his entire life is unfair too.
You guys have to reach a middle ground, but unfortunately only the two of you can come to that.
Good luck, and try, try, try to relax. Stress isn't go for us preggers, it just makes us more crazy. LOL
Edited to add: I just read my post and I wanted everyone to know that if you weren't pregnant, my advice would probably be different. I've always been the type to have friends of the opposite sex, so I think it's ok for men too as well. As far as their partners, I think a lot of times the cattyness of women make it impossible and the guys suffers. BUT, being pregnant brings about a whole new ball game, and I feel it's important for the man to realize that life is changing for him too, and his responsibility as a father starts in utero as well. This couple is young, so perhaps he doesn't realize what is at stake, but stepping up to the plate as a dad doesn't start at birth. It starts at conception. Leaving your pregnant girl friend at home while he drinks with someone she's not comfortable with is not fair. I just wanted to clarify. Counseling may also be a good idea, but only if BOTH of you go. You're about to become a mother, and you need the help and support of your partner.