If you and your boyfriend are exclusive — i.e., not dating anyone else but each other — he shouldn’t have accepted an invitation to be someone’s date to a wedding without making sure you were OK with it first. I can appreciate that people bring “dates” to weddings all the time that aren’t real dates. I was at a wedding a couple weeks ago where a woman brought her brother as a date, for example. And I can think of some circumstances where it might make sense for a woman to ask her platonic male friend to accompany her to a wedding, even if that friend has a new girlfriend — like, if the wedding is in the male friend’s town and the female friend is flying in from somewhere else and the wedding gives them a chance to catch up. But still. You are the girlfriend now — a girlfriend of five months — and you deserve some respect. The right thing to do would be for your boyfriend to give a good explanation for why he wants to be another woman’s date to a wedding in a way that makes it clear you have no reason to feel jealous or threatened (and he should also be showing appreciation to you for being so accommodating).
Also, is it just a coincidence that your boyfriend was invited to two weddings as a “plus one” or does he have tons of female friends? Is he, like, a super good-looking guy whom women would be proud to take to a wedding a show off as if he’s their date-date? Is there any chance these women like him and he’s either unaware of their feelings or, worse, likes that they’re into him? These are all just questions to consider and discuss with your boyfriend if you have concerns about the intentions of these women in his life. It’s entirely possible everything is strictly platonic and there’s no reason to worry beyond a basic concern that your boyfriend show a little more respect for your role as his girlfriend. But it’s also always good not to be naive.
Trust your boyfriend, but more importantly, trust your gut; if you feel like you have reason to worry, talk to your boyfriend about your concerns. If you don’t feel right about your boyfriend attending these weddings as other women’s dates, tell him why you feel that way and let him explain why it’s so important to him to go. If he doesn’t have a good explanation beyond, “My friends asked me!”, you need to let him know that’s not enough. If you’ve agreed to be exclusive, that means you don’t go on dates with other people unless there’s a very good reason to and that the platonic nature of the date is understood by all parties.