If you feel uncomfortable, then you shouldn't go. It's hard seeing your crush with someone else, and sometimes it's best to move on. But as kirbyfan, Pride is a wonderful event (although I've never gone to one) so you should consider attending if you haven't before.
You live close to DC or simply visiting for pride?
I live a few minutes from Richmond, VA, so a trip to DC is pretty easy and mostly depends on traffic for length.
I'm really not feeling so actively uncomfortable that I don't want to go, it's more like I'm worried about being more bothered by it than I feel at present (and have felt for the past 48 hours). It's part of why I've not given a definitive answer, and want to know when they need an answer by.
I think Pride is an experience worth having (even if I don't particularly enjoy it). I think humanizing the person he's dating also sounds like a good idea (though if the boyfriend is terrible...that might not be so good). However, spending all day with them sounds like it could be a bad thing if they tend to be PDAy or you find you're not comfortable. My suggestion: find a fourth wheel. Do you have any other (mutual?) friends you could invite (LGBT or not)? If not, what about the person whose house you're staying at—would they join you guys at the event? Even if you'd be meeting them for the first time, having that fourth person would probably be better than not.
EDIT: Something to keep in mind: you can always go to Pride another year, if you decide you would ultimately feel too uncomfortable spending a day around the couple. If you go with the fourth wheel, always keep in mind that you can split up at any point if you feel the need to do so.
A 4th person is definitely something to keep in mind, and I may ask about the person they're staying at as well. I have one or two friends that might consider going, but no mutual friends. We've only talked and hung out for a few months, and while I do know a couple of his friends, I think it's unlikely that one of those would come. It's more possible with other people he knows, but I haven't met them.
Splitting off is actually something I would likely do at some point either as a 3rd or 4th person. I'm definitely capable of enjoying some me time, and at a lengthy event such as that, I'd like some time to wander unrestricted and see what there is to see. The situation being as it is really only just insures that point.
And you're both right, if all else fails, then I can always check out an event another year.