All you can do is reason from the scriptures, which you appear to have
done. Homosexuality is as serious as bestiality, or adultery and
fornication. Actually, the Bible describes homosexuality as a greater
sin than adultery or fornication. The reasoning is that the desire for
same-sex intercourse is one "contrary to nature" according to Romans
1:24. At least with heterosexual fornication the wrong doers are acting
in harmony with the natural desire for the opposite sex.
Homosexuality is not comparable to bestiality because bestiality is a form of abuse since, unlike 2 consenting adult humans, animals aren't able to give consent to humans.
Saying that homosexuality is as serious as adultery, is like saying that heterosexuality is as serious as adultery. Adultery is the wrong of a married person (whether homosexual or heterosexual) having sex with someone other than their legally married spouse(s) (whether gay or straight).
Fornication is just sex outside of marriage - again it applies both to gays and straights.
Same-sex intercourse is only "contrary to nature" for heterosexuals - the same way that heterosexual intercourse is "contrary to nature" for homosexuals.