1 Peter 2:11-12 (ESV)
Beloved, I urge you as sojourners and exiles to abstain from the passions of the flesh, which wage war against your soul.  Keep your conduct among the Gentiles honorable, so that when they speak against you as evil doers, they may see your good deeds and glorify God on the day of visitation.
From the passions of the flesh which wage war against your soul. There are passions that wage war against your very soul. We are to abstain from these passions. By giving into these passions our soul is injured, our faith is weakened, so also our witness to others as believers in Christ.
Why is this? Paul talks of sexual sins as sins against your own body. Body and soul are not so sharply distinguished in ancient eastern thought as they are in modern western thought. And it is my gut that says this is also what Peter is talking about here. Sins sexual in nature tend to be the most degrading to a person. It is perhaps there that we see there is something very different between man and dog, and why it is so degrading to a woman to compare her to the female k9.
Oh, I have known people who have reveled in behaving like the animals they aren’t. And it tends to be destructive on their psyche. Playing Russian Roulette with std’s aside, these sins destroy a person from the inside. Even Gentiles know this instinctively, that sex should be reserved for someone you love, etc. So often though they wake up and realize they didn’t know what love was. Devastating so often to females most especially, just listen to Taylor Swift’s “Fifteen.”
And Gentiles see it. They see and notice a person with enough self respect not to engage in that sort of activity outside of marriage, male or female. They notice. They notice when you can see in another person not someone to gratify animalistic desires, but a person for whom Christ died, and thus deserves love and the respect that goes with it.