This question goes wrong with "cheat". If I don't know about it then she's cheating.
Now if you're asking "would I be OK if she asked", it's still no, not in an LTR. If it's a girlfriend, then she's branded herself fuckbuddy material. I have friends right now where the wife is carrying on with another girl on the side and it's fucked up. You put yourself onto a slippery slope when you entertain that. Every "open relationship" I've ever known has some degree of cheating, regardless of how much they swear they tell their partner everything. A chick wants to fuck. If you give her permission for a little thing, she's going to push that and find out exactly where the line is. Better to draw the line conservatively.
Now if you're asking "would I be OK if she asked", it's still no, not in an LTR. If it's a girlfriend, then she's branded herself fuckbuddy material. I have friends right now where the wife is carrying on with another girl on the side and it's fucked up. You put yourself onto a slippery slope when you entertain that. Every "open relationship" I've ever known has some degree of cheating, regardless of how much they swear they tell their partner everything. A chick wants to fuck. If you give her permission for a little thing, she's going to push that and find out exactly where the line is. Better to draw the line conservatively.