I'm going on a boys trip and not telling my gf about it
Why would you keep the trip from your girlfriend?
I might end up in a sex-related situation and I don't want to freak her out about it.
I think that honesty is always the best policy. You should tell your girlfriend about your plans so at the very least, she is not worried about you.
I don't want to disappoint her, how about I tell her after I'm back?
I feel like that is a good compromise, although I do not suggest that you act on that sex-related situation that she may get angry with if she finds out. She's your girlfriend, and I do not think she would like it if she finds out you are cheating.