The question I have is: What ultimately prevents a "taken" man from giving in to a sleazy woman's advances?
Several things. First, he may already have seen her in action and know what fate lies in store for him when she's done with him. Second, the knowledge that "if she did it to her last "conquest" (cheated on him while saying he was the only one for her, raked him over the coals financially, destroyed his marriage/relationship with someone else/etc.) she'll do it to me" will scare him off. Third, she may be too brazen about it.
Incidentally, while the pop star Pink might be labelled as sleazy because she got Britney Spears P.O.'ed when she said she wouldn't mind sleeping with Spears' then-bf Justin Timberlake, you'd also have to include actress Doris Day, who in an autobiography said if she found what she thought was the right guy for her, his marital status wouldn't matter (IIRC)--have to find the book). A pretty broad brush to paint people with.
For example, is it possible for a man already in a relationship, to turn down an attractive fellow employee who makes passes at him and lavishes attention on him?
Of course it's possible. Likely? Depends on the people involved (is he shy and insecure? Maybe this is very flattering to him. A "himbo?" He may think it's his due from life), the situation (is she drunk at the office party and boozily throwing herself at him or subtly working her way into his life?), the reason (appealing to his lust, his chivalric impulses, his willingness to help someone in trouble, and so on), the culture (of the workplace, social gathering, church, camping trip with several couples), etc. Some people are as steadfast as a mountain in situations like this, while others would make cooked spaghetti look like Superman in terms of resistance.
Can a man's sense of commitment be enough to prevent him from being a two-timer?
Sure, BUT besides her actions, many guys have to deal with the ridicule of their male buddies, who are jealous over his refusal to betray a trust his wife/SO has in him because they would trash it in a heartbeat, given the chance, which they aren't getting because she's ignoring them (for now, anyway).
Does the whole matter depend on the extent of his love and devotion to his significant other?
Not that alone. But you're right that it does come down to values (doesn't everything in life, if you stop and think about it?). Choices. What means more to the guy--this woman or (insert any other alternative here)? The competing factor might be money, self-respect, religious devotion, a promotion, an inheritance, etc.
Another thing that would help him in his struggle: other women who'd let him know he's respected for resisting and not seen as a wussy by them. Especially if he's the type of guy who has trouble relating to women in social/romantic settings, who wants to talk with them but may be painfully shy.