I always feel that its understated that there is an actual synergy going on with your neurotransmitters which actually gets you more fucked, it's like taking another drug. Amphetamines + masturbation creates such a dirty, addictive, longer lasting buzz.
I wouldn't worry about it, pretty much everyone does it with amps.
Many amphetamines make me hypersexual when I'm on them. Should I be embarrassed about it afterwards?