I have no great interest in arguing the Bible from a Christian perspective because I'm not one and I don't really care about its moral values. However since it's here, and its such a common and frightfully low effort argument, a couple things:
Levitical law doesn't apply to Christians today, the abomination of shellfish is no more or less relevent. Also there are plenty of both Christians and Jewish scholars who implore people to look at those verses with the rest of the chapter for context, since it is often talking about temple prostitution and pedastry, not everyday relationships. Things that actually contrasted against other local religious customs and practices.
Similarly 'strange flesh' isn't male flesh, and certainly not a consensual relationship between two same sexed individuals. But a mob trying to rape two angels. Comparing that to a consensual loving homosexual relationship is monstrous.
Edit: if you actually want reasons and perspective from LGBT friendly Christians there are resources like this to be had
leviticus Archives | Bible and Homosexuality
Homosexuality: Not a Sin, Not a Sickness Part II "What The Bible Does and Does Not Say..." | Religious Institute