If you knew she had a boyfriend, then you're a douche bag and should have known this could happen.

If you didn't you should tell her what happened is against your code and it can't go on.

Change workplace ?

the way you tell us, you have a grenade in hand and the clock is ticking

You’re fooling with a whore who gets drunk and cheats on her live-in boyfriend. And you work with her. I hope this is at a replaceable job and not a career workplace.

This won’t end well. I’d do what others recommend and start recording everything.

If she would cheat on him, she will certainly cheat on you.

How old are the people involved? This is some rookie mistakes. I hope there is no kid in this unfortunate situation.

Never shit where you work

Sit down with her have a chat about how you are NOT interested in a longterm relationship, maneuver her to say it WAS just a bit of fun from HER side and that it was never a serious thing.

this part is important: GET THE ENTIRE CONVERSATION ON TAPE!!!

I cannot stress this enough, get the entire convo on tape and keep it, when she accuses you of ANYTHING untoward with HR, get the recording- play it for HR and live your life free and unfettered.

then RUN!

Sexual assault charges incoming.

You screwed her life up since she wants you to fill the open role you helped create and has told the other guy to fuck off.

First rule about workplace dating. You don't stick your quill in company ink. Never should have been fucking around with people you work with.

Now that you fucked that up, you gotta ask yourself, "Did I know she was in a relationship?" In my opinion any dude that knowingly fucks another guys chick, has no honor and should be disregarded by other men.

Now you did say, it was kissing. So that might not be so bad. We'll just say you were taste testing another man's dick sucking machine. However, you did break it by doing so. Kind of rude of you.

Now that all that useless bullshit is settled and out of the way, what you really need to do is grow a set of balls and just tell it to her straight.

"I fucked up by mingling with work peeps in none work environments".

"I fucked up by kissing you because I have low moral standards, not gonna blame the drinks because that's what a bitch would do".

"I refuse to continue to fuck up so I'm telling you that I'm done with all of it."

"It's definitely me, it's also definitely you."

"You are the cheating whore, while I am a complete piece of shit."

"At least your ex dodged a bullet."

By this point things should be pretty clear to both of you that you guys each fucked up. She won't be able to rationalize it like that though because women do emotion. She'll melt down.

In the meantime you could prepare to lose your job and the real possiblity of her bringing sexual assault alligations against you. Then once you come up with a solution to that, you could always prepare for the next time your Dick starts thinking for your brain.

Is she good ma material? Don't let the opportunity pass by you. One day, you'll wake up old and there won't be any more opportunities. Not entirely true, but you get the idea. You don't want to try and make kids at age 40, if you can help it.