If the main Christianity rule is 'Love thy neighbour,' why don't a lot of Christians accept homosexuals?
For the same reason we don't accept murders, liars, thieves and other sinners while the remain in their sins. Christians understand that sin is harmful to the sinner and those around them and we love our fellow men enough that we don't want them to remain in harms way.
"For we must all appear before the judgment seat of Christ, that each one may receive the things done in the body, according to what he has done, whether good or bad. Knowing, therefore, the terror of the Lord, we persuade men" (II Corinthians 5:10-11).
"Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites, nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God. And such were some of you. But you were washed, but you were sanctified, but you were justified in the name of the Lord Jesus and by the Spirit of our God" (I Corinthians 6:9-11).
Former homosexuals we do accept and joyfully. Just as we all are former sinners in many things. The real problem is that you, and many others today, refuse to accept that homosexuality is harmful, sinful, and a matter of choice. But I can't alter truth just because you refuse to look at it.