When having sex, O'Brien and Bashir do not kiss on the mouth in a "just bros rubbing one out keeping it friendly" kind of way. But Garak and Bashir dont kiss on the mouth in a "i'm desperately in love with him and holding that back by the skin of my teeth and if he shows me an ounce of tenderness i will start crying" kind of way. Garak and O'Brien mention this to each other and both assume the other is doing the same thing with Bashir. I hope this helps.