Don't feel like an idiot.
I think what you did was a great idea. One thing that you could do is you could approach her to talk about it, and say that one of your friends came out to you as gay and you don't think that it's a problem, but that your mothers opinion of gay people is confusing you as to whether you should be bothered about it. It might allow you to get her to open up about her true opinions, without putting yourself in the line of fire.
Secondly, yes, when you're at university go to the
lgbt events, and even join the group if you like the people that are in it! As someone above said, there is no reason anyone has to know what groups you join, until you are ready to tell them! So don't feel embarassed, just allow yourself the chance to embrace it, and explore!
As for your mothers view on gay being an excuse for promiscuity... I myself have been dating my girlfriend for 2 years in a very committed relationship, she was 21 when we got together, and a virgin
whereas I have plenty of straight friends who sleep with anything that walks!!