I guess many people here mistake your daughter's boyfriend's brothers as being your children. Assuming that disciplining someone else's children are out of the question (you can have a talk with his mother about her sons' behaviour, but that's about it, I guess), I would say that you did nothing wrong of the sort. After all, what if the boyfriend decides to go a step further and have sex with your daughter just to prove he's not gay? This is also worth mentioning to your husband and it may quickly bring him to your side.

With that being said, it's best to sit your daughter and her boyfriend down and start having some serious talk with them.

First you could mention why two individuals sleeping together in a bedroom is generally a bad idea, because of temptation and what not. Depending on your attitude towards 13 year old having sex, you may also want to start talking to your daughter about well, pregnancy, safe sex, or it's illegal to engage in sex before you hit legal age. Explain why (according to your level of comfort, of course).

Secondly, talk to the boyfriend and the daughter about not having to prove anything to anyone. This is a good opportunity to impart some life lessons, for example as long as you're happy and you know what you're doing, there's no need to try to prove people wrong. The brothers' actions constitute bullying as well, and perhaps you can also teach him the best way to silence bullies is to ignore them. Just some suggestions.