It is difficult to use genetic diversity in plants as an argument for homosexual intercourse. The plants do not have homosexual intercourse and the farmer's lack of genetic diversity came from heterosexual exchange of genetic material. Homosexuality does not enforce genetic diversity on any level, as it cannot result in a new genetic consequence. Even if by choice, homosexuals must at some time, by some means engage in a crossgender exchange of genetic material in order to provide genetic diversity and strength against the rigors of a natural system. This means that the diversity that is found in homosexuality is a cultural diversity, not a genetic diversity.
However...
Cultural diversity is an important point as well in the process of the "Human Experience". Without said diversity, given social and cultural groups stagnate. The feudal system was only removed because of an injection of external ideas from there cultural viewpoints. As such homosexuality may be nessicary in society to provide progress. How it teaches us, however, may not necessarily be by positive or negative example, but instead by the development of cultural tolerance or acceptance and the path leading to it.
Oh... And BTW...
If "Seems to me that if the only way we were designed to go together was man inside woman, there wouldn't be any need for such sensitive areas in such convenient places." holds true than any argument that is pleasure with no practical ends, such as homosexual sex (which leads to nothing but pleasure, and has no practical ends,) and drug use (the only reason for getting high that I am aware of is personal pleasure,) can all be justified (and for any level of drugs, from marijuana all the way to heroine). However, you're right on the "Just because it leads to child birth it's healthy doesn't fly" point. A lot of things that are natural are wrong and could lead to larger problems (Like overpopulation.)
Lessons Learned
Mood: Ponderous