I don't think it's intended in an offensive way. Some characters just seemed like they were designed to fall for Hawke because Hawke is Hawke, rather than gender being involved at all.
Um, I'd personally call them bisexual because they're open about being bisexual. With characters that are more reserved, it's difficult to determine whether they are bisexual or whether they're simply drawn to the player character regardless of gender.
I'm sure it's not intentional, but the problem is, again, this assumes that all bisexuals have to be completely open about liking both genders, but single gender attracted people don't have to be open or obvious about it. That a gay or straight character doesn't have to go around hitting on or expressing interest in every person of that gender, or at least one before meeting the PC, but bisexuals have to announce it to be "realistic."
I'm bisexual, but I don't go around telling everyone I meet, or express interest in both men and women around every person I hang out with. Much like Merrill or Fenris, I'm actually pretty shy, a loner, have trouble opening up to people, rarely express interest in others, and the few times I do, it's exclusively for that person. Or if I comment on an attractive person or people I happen to see (like Merrill commenting on the Qunari being "easy on the eyes"), I don't feel this need to balance it out by commenting on another person or people of the opposite gender, in case the person missed that I like both genders.
But if it is considered offensive, I'll try and avoid using that sort of language in the future, sorry.
It's all right. I know it's weird saying this after the long diatribe I posted up there, but I just wanted you to understand where I'm coming from.