What causes heterosexuality?

Lots of people seem interested in causes, but for some reason this question doesn't get asked much.
Update: 1. "Normal". Nope, homo is normal too ... everywhere, all through history.
2. "God". Only works if "God" created homos too.
Update 2: 3. Since someone asked: 7 years of college.
Update 3: There are so many funny & thoughtful answers ... y'all win! ... voters can 'pick' one if they want.
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