Bioware is one of the best for this. They get a lot of flack for stuff, deservedly, but they do manage to do a few things right. The relationship systems in both Dragon Age and Mass Effect are evidence of what they do right.
When Bioware released Mass Effect in 2007, they took a lot of criticism for featuring an alien character named Liara T'soni who can be romanced by both genders. Not only because she can be with MaleShep or FemShep but also because there is a love scene.
|You slut, you.|
|OMG THE ADORABLE IS TOO MUCH|