Hell hath no FURY !!!
The simple fact is, that if you treat your wife like crap...your gonna pay, and pay big. I don't mean just money either. If you have kids, I can bet that your soon to be vindictive ex-wife is going to make sure you get to see them as little as possible. Keep this in mind and maybe being a little nicer, a little sweeter, won't seem like work after all...
However, having said that. My roomate's ex-wife got pregnant just after he tried to dump her in college, then married him, got him to pay for her education, then had an affair aound the time she was graduating, divorced him, took half of everything he had worked so hard for (he even gave up his education to get a job to support her, and his child) got remarried, and collects child support (of course he doesn't mind THAT), and limits how much he can see their child because she explained to the court that he had a drinking problem...not true of course, it's just that while they were married, he had a second job as a bartender 6 nights a week and drank a little sometimes...
In my opinion, if your girlfriend is a bitch, she's gonna be a bitch when you marry her to. If she's a pain in the ass now...imagine her in divorce court !! As far as I know, no SP has ever sued a client because he was seeing other women.
Figure out what you want...a loving, caring, supportive relationship...or merely a physical release every once in a while with someone that you'll probably never have to invest your feelings into.