I don't think you were wrong for thinking that your boyfriend wasn't invited. You made the safe assumption. I agree that you didn't have to lie, though I can see why it may have been the more comfortable choice.
However, I think it's a mistake to assume that this was a deliberate slight rather than an oversight. Unless there's bad blood between your boyfriend and the host/guest of honor, it was probably simply an error. If the host thought that they'd emailed him an invitation (or included him on yours), I don't think they would mention to you that he was invited unless you had specifically asked for clarification. A second possibility is that the host did originally intend for it to be ladies only and the guests either assumed that it was a family event or asked if they could bring their charges along. The host may have been perfectly okay with this and just said, "Sure thing!" or she may have just not wanted to say no.
My point is that while I totally understand being sad that you couldn't share this event with your sweetie, I don't think you should assume that your friends don't like your boyfriend and/or don't honor your relationship with him. You're usually treated as a social unit, which makes me think that your friends see you as one. This was really probably just a mistake.
My vote is to let this one go. If one of the hosts or the GOH had asked where he was, I think a good response would have been, "When I got the invitation, I assumed it was a ladies' only event. Too bad, I know he would have enjoyed celebrating!" But I don't think it would have been good to say that he wasn't invited unless you knew for a fact that had been the intention. (Even in that case, doing so likely would have put a bit of a damper on everyone's good mood.)
In the future, I think you can make polite inquiries if you aren't sure about an event. If you want to go subtle, use the singular first person a bit more heavily than usual. If the host asks after boyfriend, say, "Oh, is he invited, too? He didn't mention getting an email. I'll tell him to check his spam folder. I'm sure he'd love to come. I'll have him call you."