Thats really hot, and there is no problem to keep them at least for some days and then give it back.
I often wear my flatmates underwear when I know he will be out for some time. He is a hot straight guy, very fit and with the biggest **** I have ever seen. We do laundry together, so its kind of easy to take some pics and wear his boxers for a while (as I am right now, ******* hard as every time I wear them). The most amazing thing is that his boxers on the dirty basket are almost always with cum ( I believe he always cleans himself with own underwear). God, it makes me so horny to smell,
