I don't remember Ax ever sitting on Marco's lap, or David spying on Rachel in the shower.
In #35 Ax sits on Marco's lap when they are trying to force William Roger Tennant to go nuts at a banquet. Marco and Ax go to sit down and there's only one seat so Marco makes him sit down on his lap to keep Ax out of trouble.
The shower thing is from #22. After Rachel talked to her sister Jordan, David starts speaking to her in morph (most likely an insect in the room with her) and ends by telling her to enjoy her shower (which she decides to skip).
I can't see Tobias or any of the others being gay. I could maybe see some of them doing drugs or drinking, but I still wouldn't like it. Jake might have had a lot on his shoulders, but he knew the others relied upon him and he wouldn't want to risk getting the others hurt by compromising his judgement with drugs. Maybe if he'd stayed human Tobias might have, but as a hawk I don't see it happening.
I definitely think Rachel and Tobias should have been more intimate. Tobias might be shy but Rachel is much more confident. Cassie/Jake might have taken longer, but I think that would still have turned into something more later on.
Marco was too level-headed to go for drugs, but as a vice perhaps he'd resort to sleeping around? He's often portrayed as a failing ladies man, but in a more mature environment maybe that would have changed.