I don't think you can count being raped as part of someone's sexuality. If a straight guy is raped by a guy, I wouldn't call the victim bisexual.
Merrill had nothing. She was the virgin romance option, like Alistair in Origins.
While I think it's safe to say that Fenris was probably sexually involved with Danarius (and possibly Hadriana as well), I'm not sure if that can be attributed to his sexuality. He was a slave and it was more likely a forced sexual relationship. He does end up sleeping with Isabela if neither are romanced, so we know that.
Merrill really didn't give much to go on, but some people have suggested that her line, "They were easy on the eyes" (about the qunari) to indicate that she is attracted to muscular males.
However, the reality is that neither really have a canonically defined sexuality. I tend to think that they are bisexual. It would be really weird to have 7 LI's in the DA games that have defined sexualities and two that are playersexual. I'm just not sure why they would do that.