Asexuality has nothing to do with being nervous about or liking sex. Some asexuals enjoy sex. There's nothing that suggests Taichi is asexual. Simply nervous about sex. Kohei liked him first, and he's more straightforward about his feelings. When it comes to feelings, even with simply their romantic relationship, Taichi has been shown to be easily flustered. I didn't really think Taichi was forcing himself to please Kohei, either. So yeah, no evidence whatsoever that Taichi is asexual. Just shy when it comes to intimacy. Coming from an ace person. Asexuality is all about sexual attraction, not about nervousness when it comes to sex.
He doesn't really show interest in physical stuff, is constantly blushing & getting flustered when the topic of kissing is brought up,... That scene in Kouhei's bedroom when Kouhei goes too fast really showed the difference between them, I feel like at this point he's just forcing himself to please Kouhei.