Stained underwear, can someone explain the logic of men having precum on their pants

Hi all, I've stumbled across this forum as I too am experiencing the same dilemma. I find white stains on my bf underwear some days. Sometimes I'll notice it when he tries to rub it off. For those of you who says Its not normal to be choking your bf underwear, well I think when you know something is up and you have your own suspicion you would probably play detective like most women on this forum who are experiencing the same problem . It has been an ongoing problem for us...he gets really defensive and annoyed when I question him about it, sometimes he turns it around and make it sound like that I'm the problem as its all in my head, most occasions he would walk out and move back to his mum for months then ends up moving back in again after. He has those stains (willy drips) as well when he goes out on a night out with his friends, also he's got a habit of disappearing when he's out with his friends.... I know what the problem is....he cheats, 100%! but I guess I'm too soft and too stupid to just put an end to my ongoing suffering of mental and emotional breakdown...

is there anyone who has medical background that can explain the logic of men having pre-cum on their pants.