I think there are several possible explanations for the David/Drew situation:
1) The simplest explanation is that the affair is exactly what it appears to be. For years Drew was sending sexual emails to David and he was responding eagerly. Then they would meet up during David's "business trips" and have sex.
2) It could be that we're somehow misinterpreting the emails. Perhaps David had a legitimate reason to meet up with Drew? I don't think we know what David's job was, maybe he was somehow involved with Drew's & Hunter's operation? Maybe the emails were some kind of bizarre inside jokes between Drew and David? Maybe Drew was acting as a therapist for David, and the "inside of me" comment was a joke about Drew being inside David's mind? Maybe Drew's topless photo was just to show David the progress of Drew's transition to a woman? Maybe David was interested in transitioning too?
3) The emails could be altered or fabricated to frame/defame David. Remember who gave Sterling the emails: Kevin, who has been untrustworthy and may have been clever enough to pull off the deception. If that were the case, I wonder why Kevin would bother doing that? I don't see how it would benefit him. If he had tried to blackmail David, surely David could disprove fake emails? Or if the emails were created after David's (supposed) death, what for? I doubt revealing them to Ophelia would've sent her into Kevin's arms. The only other thing I can think of seems too convoluted: that Kevin was somehow conspiring with Hunter and/or Drew, and faked those emails in order to manipulate Ophelia for their purposes.