If someone told me they were in love I'd say, "Who's the lucky [pronoun that matches the gender that I know you prefer]."
If it's an unknown-preference, yeah, I'd probably default to straight if only because more people are straight than they are gay, if corrected I would apologize, ask who the lucky [person] is and make no further issue of it.
A guy at work the otherday started talking about his boyfriend and I made no reaction to it and talked to him as if he was just anyone else talking about their SO.
It really doesn't matter at all to me on whether or not someone is gay or straight. Yeah, I'm going to probably default to straight unless otherwise informed if only because I'm playing the odds and it's "safer" to assume someone is straight (no matter how they act/behave) than just to assume they're gay due to "tells" like mannerisms, use of language, etc. I know a guy who's straight and he like a slightly dialed down version of Jack from "Will and Grace." He's been married for several years, has had many girlfriends, and has kids as well as being in his late 40s. I also know of a guy who's pretty much GI Joe mixed with Jack Bauer and he's gay.
So assuming someone is gay based on how they act is, dumb.
Unless otherwise informed, yeah, I'm going with straight. If I find out they're gay, I was wrong. Life moves on without any change in my impression of them.