Attraction and rejection

This weekend it came to my attention that someone may be attracted to me who very much shouldn't be.

In the past, I have been very naive about matters such as this. I run under the assumption that, with the shared knowledge that I am in a committed relationship (and what I presume is also the shared knowledge that I believe in monogamy), that the boundaries are solid enough that I no longer have to worry about crossings.

This strategy has, unfortunately, come to bite me in the ass on occasion. For instance, I once let a male friend who knew of my relationship, spend the night on my couch after losing his car keys, only to have to push away a kiss attempt. Had I been smart, and perhaps a little more skeptical, I would have seen it coming, but I presumed he would respect the boundaries I thought were clear.

This leads me to wonder what is the appropriate strategy for dealing with such hunches if stating your commitment to your significant other is not sufficient. Am I to cut off any communications with anyone who might plausibly be attracted to me? Am I supposed to state multiple times over how I would never cheat before they express any explicit interest? Considering I am hardly beating off men with a stick at every turn, I really do loathe to assume that all men I come into contact with are secretely plotting entrance into my pants, especially when I have several long-time close male friends who have never attempted any such thing.

I read one article which suggested that human beings, in general, are very bad at rejection. We so despise the notion of hurting another's feelings that we sugarcoat everything.
For instance, one time, I thought I had made a very clear rejection speech, and was perplexed as to why it didn't seem to phase the fellow as much as I'd anticipated. A friend later told me, that by virtue of including the whole "You seem like a really great guy who I'm sure a lot of girls would love to be with with, but..." spiel, he probably assumed I was one of those girls, while I thought I was making it clear I wasn't in that category. Strike one against sugarcoating!

I also think these boundaries may be especially to establish for women. We have this whole culture of "nice-ness", especially when we are so petrified (at least I am!) of falling into the seemingly only other category... that of bitch. As such, I have been witness to many friends continuing to hold conversations with utter slimy creeps because they don't want to seem mean, or even hold onto friendships with men who clearly have only one motivation in mind because they are apparently, underneath it all, "really nice guys". I also do know that my gender can be experts in leading men on, which may contribute to some of this. However, lest you think I am being too extreme in this perceived dichotomization of women by some men as "interested" or "bitches" , let me just say that one time I kindly told a guy at a bar that I wouldn't dance with him because I had a boyfriend, he threw an utter fit, asked me why I would even come to a bar in the first place, and followed me around for a good proportion of the night screaming profanities at me.

Tangents and stereotypes aside... what are the best ways of setting these boundaries?