If a guy agrees to a monogamous relationship and then "outsources", that would be a problem. I would like to think most men are capable of being in a monogamous relationship, but the statistics probably (I don't have them off hand) would show that both sexes have problems with being monogamous. I haven't had any problems being monogamous, myself, but I know plenty of guys who do "outsource". They just can't keep it in their pants. Just because someone is a slut, doesn't mean they can't be your slut, and your slut alone.
See, I more think of constantly shagging just one person as monogamous nymphomania, not sluttiness. I'll break down my understanding of some terms which could potentially be seen as derrogatory:
Slut= Sleeps around/has a lot of past and/or present sexual partners
Easy= Hops into bed relatively indiscriminantly
Fast= Hops into bed fast (duh)
Whore= This one is more vague, and is really more likely to be used as a way to vaguely bash a sexually active female, especialyl one whose choice in sexual partner upsets you
Nympho= Wants constant sex
Anyone disagree with the definitions/have anything to add? (And by that, I mean, do you think the defintions are correct and do you have others, not do you think they apply to me. :p Be interesting to see who thinks they apply to them, though. Maybe I'll start a thread or poll about it, when more definitions have been generated/validated.)