(spoiler)
...since the family has to decide which one of their member to sacrifice, the answer would be too obvious in a conventional male/female couple with a young daughter, as the traditional role model for the father would be to protect the females in his family. More suspense with a same sex couple (either male or female).
Both good points.
But I suppose in the case of this film's same sex couple, where both men are of approximately the same build and age, which one is the "weaker" is entirely a matter of their personality (which you get to discover as the story unfolds), rather than biology (not saying some females are not physically stronger than some males; but most are not, and it would be immediately obvious from the beginning of the film).
What you said is a pretty interesting way of looking at the film, but that's only looking at it in a boring traditional way. That's because the sacrifice is not dependent on who's considered the weakest or the strongest---it's the fact that the couple was being told that a sacrifice was being required at all in the first place, which is disturbing enough.
Also because it gave them a reason to think they were being targeted by homophobic religious fanatics. What other reason could they come up with when it seemed completely arbitrary that they were the ones chosen?