Because the media has convinced us this is so. Because, as seen in the media, there is no struggle, no pressure in regards heterosexual attraction. Of course, the world's standard, even among church goers and homophobes is that heterosexual activity before/without marriage is normal and acceptable.
Because homophobia is a real thing with sometimes violent consequences. Nobody has ever been lynched, beat, or drug behind a car because they wrte heterosexual. Straight bashing is not a thing but I would be confident in saying some Witnesses have engaged in gay bashing even if only in word.
Because no Witness ever says "ew, they are in an unmarried heterosexual relationship? Gross! I would never want to be in the same restroom with that person.
Feeling differently about a homosexual couple than a heterosexual couple is understandable. Heterosexual sex is how we were designed. All a heterosexual couple need do in order to be approved by Jehovah is get married. A homosexual couple does not have that option. Their relationship must end if they ever want Jehovah's approval.
It is harder for a homosexual to become a Witness than a heterosexual. Yes, both must resist immoral urges and desires, but the homosexual must reject the entire lifestyle.
While not directed at homosexuals, Jesus did say it's harder for some to come to Jehovah than others.
(Matthew 19:24) 24 Again I say to you, it is easier for a camel to get through a needle’s eye than for a rich man to enter the Kingdom of God.”
https://www.jw.org/finder?wtlocale=E&pub=nwtsty&srctype=wol&bible=40019024&srcid=share