Years ago on Y&R, Adam Newman (heterosexual) had sex with his gay friend to keep his misdeeds from being unconverted.
My feelings here is, at the very least, Marco is bi-sexual. But I would place money on him being a gay man. This is Lucas' next relationship. That being said, yes I think he will be up to something and so I see this playing out a few ways:
A. Marco is up to something and he and Lucas enter into a relationship. Marco catches real feelings for Lucas. They have bumps but their attraction and chemistry will win out.
B. Marco will break Lucas' heart and Brad will be there to pick up the pieces.
C. Brad will expose Marco, thus playing the hero for Lucas and they will give things another try.
Marco did reciprocate. He had his left hand on Lucas as they were kissing. I agree that Lucas, initiated it and was more into it but not only did Marco not hate it, he appeared turned on in the moment.