It’s not a stupid question. The only way you can understand other people’s feelings is by asking them to explain.
There are two ways to explain it: in terms of how men feel, and in terms of why they feel that way. To answer the first way, well, PaulinVA gave the only answer there really is. It’s all about hormones, and if you don’t have those hormones, you can never know what it feels like to someone who does.
For the second kind of answer (the ‘why’) it is all about survival. In many species, the male has a powerful impulse to mate with as many females as possible. Males will compete with each other, even fight to the death, in order to win the right to mate. This is nature’s mechanism for ensuring the survival of the species - the male who is strongest, fastest, etc. gets to pass on his genetic material to the next generation.
Human males also have the powerful impulse to mate with as many females as possible. Of course they don’t think about it in those terms. When a guy sees a good-looking woman in a bikini, he doesn’t think “Gosh I need to pass on my DNA”. All he knows is that he feels sexually aroused when he sees her. This is why men find it so difficult to stop looking at an attractive woman, especially if her clothes are revealing. By the simple fact of seeing her, that drive to reproduce has taken over and he is already in the first stages of arousal.
Of course if the man is a Christian (or has had a good upbringing) he knows that he needs to restrain his biological impulses and so forth. I’m not trying to reduce everything to biology. But you said you wanted to know why it is so hard for men to abstain, so there you go