My high school crush, whom I was too nervous to ask out, got touchy-feely with me at a recent party and seemed really into me, though she has had a boyfriend for the past 2 years.

My high school crush, whom I was too nervous to ask out, got touchy-feely with me at a recent party and seemed really into me, though she has had a boyfriend for the past 2 years. What should I do? I don't want to invade on her relationship, but I still really like her.
My high school crush, whom I was too nervous to ask out, got touchy-feely with me at a recent party and seemed really into me, though she has had a boyfriend for the past 2 years. What should I do? I don’t want to invade on her relationship, but I still really like her.

Be straightforward about this. Your reasons and language about it are good: “I really like you, but you have been with {boyfriend} for 2 years; I don’t want to invade your relationship. What are your thoughts?” Be prepared to find out she doesn’t think there is anything wrong with having a boyfriend and having some casual fun with you. In that case, you need to decide what your stance on that is.