Based on the description it sounds like pre ejaculation. (Semen). Keep in mind that as man ages sometimes even things can cause him to seep some semen fluids so this isn't necessarily any sign of infidelity. Some men seep pre semen for many reasons. Sometimes excitement will do it and sometimes it for no other reason then he has taken a small nap and had some seepage from that. The way his clothing rubs can cause this as well.