When I first started having sex, I had one hell of a time orgasming during sex. In my case, it was because I was really hyper-vigilant (childhood abuse issue, not going into it, here.) and every time there was an unexpected sound, I'd lose focus and get distracted by whatever the sound was. (I'd still keep fucking, but I'd switch my attention to whatever was happening.)
Almost there...a bird chirped. Aaaaand back to the plateau phase. I'd come 'round again. Nearly there. So...so close....The garbage truck starts hammering cans, outside. Try to keep the edge...aaaand back to plateau. Keep going, comes 'round again...a snatch of conversation comes in through the open window. Damn, damn, damn! Keep going.
My girlfriend (past tense,) really liked that it took me so long. We'd end-up in marathon fucking sessions which would last between one and two hours (which are possible, when one's nineteen, but not later in life, so I'm glad I go over that problem.) Sometimes, though, one or both of us would start to get rubbed raw, just from the duration. She'd come like gangbusters, but eventually, she'd get tired. I'd pull muscles, just from working them too long, fucking, trying to get off. It...wasn't a good thing.
Sometimes, I'd get tired to the point I couldn't keep going, but she'd have a fit if I didn't come -- she'd feel as if she'd failed, somehow. So I learned to fake an orgasm. (Yes, guys can fake 'em too. It isn't difficult, really.) I felt as if I were cheating, or at least lying, when I did, though, so generally, I just kept going, until I finally came.
Luckily, with practice, I learned how to hold my focus properly, and since then I've had no problem whatsoever, in that department.
Bit it was hell, that first year or so. I don't miss those days at all.
[No, this isn't a fantasy. It happened, years ago, and it was annoying as hell and often painful. I wouldn't wish it upon anyone. 'Nuff said.]