I think that heterosexuality is culturally and/or socially taught. I don't know if that is covered under your question...but it seems to me that the level of cross-gender pairing and avoidance of same sex partnering has got to come from somewhere. It doesn't seem like it can be nature/biological, so it seems me like it would have to be cultural/taught.
Les
For those who think sexual orientation (Gay, Lesbian, Straight, Bisexual) is culturally or socially learned, I'd be interested in hearing why you think that.