As for Drully, I'm sure Drew didn't expect anything to happen at first; he would have thought Tully was out of bounds, being a 'lesbian' and in a relationship. But then when Tully really started with the flirting etc., he would have thought "well I guess she wouldn't be doing this if she thought her girlfriend would mind that much". Drew got signals from Tully that what they were doing was ok. So I'd put most of the blame on Tully in that situation (and by the way, Tully was my fave, even throughout the whole Drully saga).
I know that other HMs said that Clawson was mutual and ongoing, and I think Lawson is mildly attracted to Cat, but it seems pretty clear to me that Cat is the one instigating the whole thing. Lawson doesn't want to get too close because he likes her a bit and he doesn't want to do anything he'll regret, but Cat invited him to the Sanctuary and insisted on him sleeping in her bed, and then she seemed to be cuddling up to him. Cat is in love and Lawson can't get away, so he lets himself be tempted.