This is not homosexual behavior. This is auto-eroticism. The only way one could show this behavior was homosexual is if one could demonstrate that the dog was desiring the male gender of the animal he was mounting. One can't conclude from the observed activity alone that any animal has homosexual desires . This is just another example of a hasty judgment.
Christians are also capable of hasty, unbalanced judgments.
They often hold points of view that are not well justified or even are consonant with the full counsel of the Scriptures. I get frustrated when people proof-text their pet point of view, but are unwilling to look at any other texts against their view. They just want to keep citing their pet verses, even when those verses are equivocal. I've faced this in my teaching on Decision Making and the Will of God. For those who disagree with my particular approach, please take the time to look at my biblical assessment and critique the verses.