Early Christian repression of homosexuality was not a reaction to Roman sexual mores, but simply was Roman sexual mores. Romans penalized adult male homosexuality because it subverted the structure of society - since all freemen are equals, they should treat each other as such in a harmonious society. If one is becoming a receiver in a relationship, this demeans their status; and is a crime against society (and is punishable). This was the point of the Lex Scantinia, "...it was essentially a rule to police the masculine nature of a Roman citizen by enforcing that a freeborn takes the “top” or “active” role in sex." (1) A recent answer by u/Semajijohn more thoroughly covers this topic in general here.
But some people did go through with male-male marriages, because there will always be those who buck the system. Even if it was "disapproved of" by polite society. The Roman M. V. Martialis said that male marriage was "something that occurs not infrequently, although they disapprove of it." (1) These marriages were done even when they had no legal standing, as noted by u/WelfOnTheShelf in their answer about Roman homosexuality here. One such "illegal" gay wedding is shown in Juvenal's Satire 2, but this included guests and the usual marriage celebrations. u/boo_cait in their answer about Roman homosexuality here cites Amy Richlin in thinking this event is "evidence of a 'gay' subculture in Rome" (in the words of boo_cait). But fundamentally, it was unnatural: it disrupted hierarchy (which was natural of course).
This general view about homosexuality was the opinion of Romans when they stepped into a flowing river and ceremonially died: baptism. In spiritually dying, they became a true follower of Christ and now they would meet him after their bodily death. “They” could live forever (in their parallel spirit form) now. What usually happened was that the spirit form was breath/air, and so was dispersed at death into the material world just as the body decomposed. Instead, they would ascend upwards to the good Other world “above the sky.” Once reached, they would rest with the spirits and with culture heroes such as Christ. (I’ve been reading about cosmologies lately so I just think it’s fun to phrase these beliefs in the general terms used to describe such “anthropological studies.”) But after stepping out of that river and rejoining Roman life, those people continued to believe that homosexuality was unnatural. At first, this was only the opinions of incidental Romans, but eventually this was the opinion of the Emperors.
As Christianity was gaining more and more converts in the Roman empire it was subdividing into groups who accepted the Jewish Torah and others who only partially accepted it or rejected it entirely. Roman homophobes who accepted these books would confirm their Greco-Roman biases by citing Leviticus 18:22 and 20:13. In the Jewish tradition male-male homosexuality was a to’ebah, the highest type of abomination. (3) As with other “unnatural” crimes, it should be punished with death. Romano-christians could point to Deuteronomy 23:17 which says “daughters of Israel” shouldn’t be a temple prostitute and neither should “sons of Israel” (for male prostitutes this would mean servicing other men).
Romans who didn’t accept the Torah had to rely on other sources to confirm their biases. These Romans often loved Paul-Saul, whose writings (ca. 50’s CE) say just about the same thing. In Romans 1, Paul chastises wicked people who are foremost idolaters,
[They] exchanged the glory of the immortal God for images made to look like a moral human...Because of this [idoltry], God gave them over to shameful lusts. Even their women exchanged natural sexual relations for unnatural ones. In the same way, the men also abandoned natural relations with women and were inflamed with lust for one another. Men committed shameful acts with other men, and received in themselves the due penalty for their error...They have become filled with every kind of wickedness, evil, greed, and depravity. They are full of envy, murder, strife, deceit, and malice.
So this isn’t just a rejection of homosexuals as such, but they were lumped into a general group of “wicked people.” These people didn’t just do one kind of sin but did all of them, this was quite the slippery slope. But Paul, himself raised a religious Jewish man prior to his conversion, was inheriting and transmuting his own biases into his new ideology.
Some Christians rejected the Torah as either not necessary (Marcionites) or the work of the evil god of materiality (gnostics). For those gnostic Romano-Christians who could not cite Leviticus or Deuteronomy, they had to add homophobia to their own stories. In the gnostic Gospel of Judas Iscariot (likely from the early-mid 100’s) the disciples have a collective vision of the Jewish priests sacrificing and doing their priestly duties in the temple. But on closer inspection, they are performing unholy and unnatural rites instead. This is a pointed jab at those proto-orthodox Romano-christians who rely on priests, bishops, and deacons for spiritual guidance. The gnostic response is that their rituals actually celebrate the commands of the evil god of materiality and are thus a perversion of truth. Homosexuality is included in this list.
[Some priests]...sacrifice their own children, others their wives...some sleep with men...some commit a multitude of sins and deeds of lawlessness.
(4)