Why were those guys teasing Dr Harford?

Who say they was teasing? Some guys is proud of they dumps.

Those guys were probably there as another stab at Bill's sexual dominance and ego.

Young, wild, macho idiots often accuse any man who is well dressed and well groomed of being queer, and gay-bashing is one of their main hobbies. Nothing unusual about that. Their purpose in the movie is simply to provide another attack on Bill's sexual self-esteem: first his wife tells him she would have dumped him for a man she never even spoke to if he'd indicated that he wanted her, and then he's taunted as a homosexual, just when his male sexual ego is already very fragile and under attack from his own imagination. The combination of events is what helps to propel him to seek a sexual encounter, to reassure himself of his sexual identity.

In reality they were just drunk idiots. But for the movie its another shot at Bill's ego. He's already been beaten down and probably feels like less of a man. The guys teasing him didnt help.

"I've taken dumps that were bigger than you!"

It’s a bizarre insult to launch at Dr Bill. While Cruise is a shorter than average guy, he’s hardly a dwarf or a ‘squirt’ - someone remarkably small.

Some people here seem to be conflating the insult with the gay-bashing. That’s even more bizarre. There doesn’t seem to be any correlation between shortness and homosexuality. ‘Gays - smaller than dumps’ isn't a thing AFAIK.

It’s a weird scene but, then again, the whole film is weird and it’s clearly there to show Bill’s masculinity taking another hit after his wife ripped his balls off with her Naval Officer fantasy.

because they are a pack of hormonal angry teenagers and he is a random adult walking alone at night