I don't blame you for still wanting the sex, i did the same thing. It just makes it more complicated. You know you're shaing her with another guy now, right? She sleeps naked and didn't bring anything to sleep at the other place and stayed there all night, didn't get home till afternoon - so obvious like you say.
You're appeasing her by having sex. If you have to do hit, treat her like the slut she is being right now. Slap her around a little get physical during sex (not to the point of hurting her), but letting her know who da man is.
She'll probably say - "That's hit, that was what was missing from our relationship, you treating me like the pig that i am in bed". LOL, you gotta laugh a little about these things, because if you don't, it might just kill ya.