Too confused.

Welcome, Matt!

It's got to be pretty upsetting and perhaps a bit confusing to you to build yourself up for this and then not have it go as you wanted. And I think it's awesome that you feel comfortable enough to talk about it.

First, this is probably one of the most common outcomes of a first-time hookup for someone just exploring their sexual orientation. There are dozens of factors that can play into this: nervousness is usually front-and-center, and that can be a huge boner killer.

Secondarily, this was an older man... so unless you are actually sexually attracted to older men, there may have been some unconscious sense of simply not having any real attraction to him. Just because you enjoy sucking him off doesn't necessarily mean it will actually be arousing for you.

Third, since you identify as "straight but curious", if you think about it... doing this is taking a big step toward saying "Uh, maybe I'm not so straight." And that's, for almost everyone, a huge anxiety-producer. It's one thing to sit at home and spank it to gay porn; there are ways we can rationalize in our minds that we're still straight if we do that. It's much harder to do when you're sucking some guy's dick, and so that can create some confusion and anxiety, which, in turn, can also be a boner-killer.

As I said, many, many guys -- often even with people they're super attracted to -- find themselves in your situation. Physical sexual arousal is a complex interaction of psychological, physiological, and emotional, and all three have to be in line for most people before they can get physically aroused. And, of course, as soon as you're in the situation and can't get hard, all that happens is you start putting pressure on yourself, and feeling embarrassed, and that, in turn, makes it even harder to get aroused.

So the best thing you can do is simply relax, acknowledge that this happens, it happens a lot to people in their first sexual experiences, and it's nothing to be embarrassed, ashamed, or uncomfortable about. If this guy wasn't particularly attractive or arousing to you, it's worth waiting to find someone closer to what you like in age or appearance... I'm not a fan of hookup apps, but there are certainly a plethora of people of all types available for the choosing on them.

One other suggestion: Try giving up porn for a month. Masturbate without it, and create fantasies in your head. Take your time, don't rush through it. Make the fantasies as detailed as you want, with whomever you want. You will probably find, the first times you do this, that it may take a while, because you've probably conditioned yourself to masturbate only with porn. But if you put in the effort, what you'll find is that you can have a much richer experience based on your own fantasies, and this will also more closely mirror the experience of being with another guy.

Finally, while doing the masturbation-without-porn thing, you can try alternating between fantasies with guys and fantasies with girls. Porn isn't a good indicator of sexual orientation, but masturbation fantasies are. And so, if you find that it is mostly or entirely guys that you're fantasizing about, then thinking about accepting the idea that you're gay, or on the gay side of bi, is also a piece of this puzzle.