First of all, your demographic doesn't match with Tenchi's. He's 14-17 (I think 16?)
If you notice in the series, he really only has about two friends and lives with his father and grandpa (no mother anymore, remember).
He's not used to people, let alone the kind that make no sense to him (the female variety).
The advances the girls make scare the hell out of him because he doesn't know how to react. He doesn't know the proper way to show affection (if he even is attracted to any of them).
Speaking from experience, when I was his age, my "skills" with women were...less than spectacular. I had many more guy friends than female-type friends. I was oblivious to all dropped hints, and I was in shock for about three days after my first kiss.
Tenchi's a shy, inexperienced little boy.
You'll notice that he DOES grow up as the series moves along though.
Not everyone matures at the same speed. Labeling someone as a homosexual based on their relative maturity is quite unfair.