Your link specically says that "In Roman Catholicism, homosexual acts are considered contrary to natural law and sinful..." and whoever told you that sin is only in the heart, not in the act, lied.
The Roman Catholic issue is about knowingly letting homosexual men into the priesthood, men who have a desire for men,
not men who practice homosexual
acts. (Homosexual sin or not a sin, it's still a very dangerous thing to do, as proven by the molestation scandal.) It's like you have no understanding of the issues.
How can sin be in the act if there's no sinful desire behind the act? And, if there's sinful desire that makes the act a sin, then it's the desire that's really the sin.