Then again, it may be nothing more than your husband thinking about your naked body, seeing your vagina in his mind, that causes him to have a super hard erection during his work day. May be he has been shopping for some really sexy panties for you, or nightie, and the thought just made him have a very hard erection and he had pre cum in his shorts.
It is very normal... men think about sex 80% of their waking hours compared to most women.
Some excellent advice there - yes - men get turned on over the slightest thing and many (including myself) leak precum juice without getting erect, merely by thinking about sexy things. I often have white stains on my underwear from precum - it doesnt mean I have been unfaithful, just that I am a normal man with a good healthy sexual appetite.
Are you keeping him sexually satisfied? - That could be why he is staining his pants - having sexy thoughts all the time!
To whomever would be offended by your significant other checking your underwear - YES I CHECK MY HUSBANDS UNDERWEAR!! I've done this man's laundry for over 18 years and clearly notice when things look different and lately have never seen THIS MUCH semen on his undies, or this often. No other symptoms suggesting prostatitis... so I have to wonder who he is oogling, cause it sure as sh*t ain't me. Most women don't do this as a usual daily task. Then comes the day when we catch our partner in an affair. Sex life was great, but months before discovery, sex dropped like a rock. When I found out about his affair, I was devastated, and still am very uncertain. I got the whole "it was only an emotional affair (geeze! insert standard answer here, right?), ... that turned out to be a bunch of c**p. I immediately went to the Dr and was tested for everything STD. Luckily, everthing came back negative except for the HPV - It was "inconclusie and so i have to get tested every 6 months. A friend of mine went through a similar situation, but she unfortunately had to undergo cancer treatment due to the HPV her husband acquired from his w****, and shared with his wife. We've been through counseling and don't think it did anything but postpone his behavior. I've since been told by people who've known him for over 15 years that this is the 4th affair he's had since we've been married. Nice that everyone knew yet they were all willing to let me catch a disease that could kill me, because they don't want to get involved. You'd think I'm some ugly naggy b***h... completely the opposite, but back to my point. I was the last to know, and absolutely am not going to put myself in a blind trust situation ever again.