Speaking as someone who often interviews candidates, I can assure you, it's not legal in any state I've ever worked in as a manager to ask about sexual orientation during an interview. duglyduckling is right about all that stuff.
Some of that is actually really annoying in some sense, because you can't ask if a candidate is married or partnered or has kids even in the context of trying to figure out a relocation budget. You have to dance around it, like, "Uh, so how many people will we be moving out here?
" and go from there. But I understand why the laws are in place, for sure.
I did ask the woman who runs HR at my current studio about domestic partners w/r/t the health insurance when I started here. To my delight she grinned and said, "That's going into effect in a couple months. The wording is very lenient... because I need it to be, myself."
Anyway, if anyone asks you about sexual orientation during an interview, I'd politely dodge the question by informing them, "Hey, just so you know, it's actually illegal to ask that during an interview!" They'll probably be surprised to hear it, but they certainly shouldn't press the issue after that.