There are many reasons not to get involved with someone who is otherwise monogamously committed to another person. It's not because you're a slut destined to ever be unhappy, or because you're betraying some sisterhoodly duty to prevent someone else's man-child from betraying her. It is, however, because, regardless of what your relationship or dating goals are, you're likely setting yourself up for failure (and a whole raft of shit, which you know if you've been reading comment thread).
The person with whom you have decided to have a monogamous relationship owes you fidelity, as you owe that person the fidelity you also promised. If that person breaks that promise, I can guarantee one thing: he or she made the choice to do so.
There, I said it. No fairy godmother is going to come down and give you fancy One-seeking slippers, there's not some other higher-power-created other half of you waiting equally wistfully for you to walk into his or her life, and there's no ultimate, perfect person out there that, if you make one mistake or break one undefined rule, you'll fuck it up with and thereby end up alone.
The "three-date rule" is stupid. So's the five-date rule, the six-month rule and any other rule that someone's told you should govern the time in your relationship that you choose to engage in physical intimacy. The problem is that, in sorting through all the messages of when you should or should not submit to what the other person apparently wants from the get-go, not enough people get around to considering what feels right to them.
I always felt weird about the dynamic of men paying for dates by rote: even when I didn't have much money, my preference was to go somewhere I could afford to split or find something to do that we could both afford. But it took a guy's behavior to really enable me to explain in graphic detail why I was always so bothered by it.
I hate the term "cougar." I mean, I also hate "MILF" and anything else that attempts to define older women or those that have have had children as inherently unfuckable—and thus the rare few supposedly worthy of placing a man's dick in as special—because, damn it, I have no intention of leaving my sexual self in the same dustbin as the idealism of my pre-recession twenties. But I hate cougar because there is no predatory animal to whom men that devote themselves to exclusively fucking younger women are compared: men are, of course, going to want to fuck younger women, and yet older women are supposed to content themselves with ever older men. Fuck that shit.
There's nothing like trying to create a 10-point guide of how not to be a disrespectful immature jerk when dumping someone to make me think of the far more than 10 disrespectful immature jerks who have dumped me. My pain will hopefully be someone else's gain.
When I stopped being a "girl," and stopped dating "boys," it felt weirder and weirder to be somebody's "girlfriend" or have a "boyfriend." But, man, do people hate having to alter their vocabulary to match your relationship.