"Straight" is a fantasy for some gay men. They'll tell themselves their trick is "straight" even after said trick has spread open wide and taken a cock while screaming in pleasure at the top of his lungs. On the other hand, "straight" men hold onto that label out of some fear that the slightest attraction to men, whether acted on or not, will tarnish their manhood. Straight men live in fear, like little boys, of being called a sissy or fag. We socialize men to be so fucked up about their sexuality that they need to hold onto the label "straight" even as their cock is in another man's ass.
While I don't think any particular sexual act, even if repeated, makes a person gay, straight or bi, I do think that both gays and straights have a mostly unhealthy psychological need for these tired, antiquated labels.
The labels aren't real. They never have been and never will be. It's a fantasy that's given social meaning. That's all.