Underware - boyshorts (Sloggi or Calvin Kline preferred brands) or bikini style knickers, cotton. I rarely wear a bra - I don't need one - though I do wear sports bras for gym sessions.
Cheating - No, I have never cheated though, to be fair, I have rarely been in an exclusive relationship.
Sexual fantasy - that's a difficult one. I don't have outlandish fantasies, mostly I "fantasise" about being able to spend more time with my girlfriend, who lives in Amsterdam.