Here is a solid article about this.
The Shellfish and polyester laws are both ceremonial, or cleanliness, laws. The Jews had to follow these in order to be clean before God.
Abstaining from homosexuality is not only part of the cleanliness law, but part of the moral law. The ceremonial law no longer applies. The moral law always does.
How do we know this? Simple hermeneutics. In the New Testament, we see that the ceremonial law is fulfilled in Jesus' sacrifice on the cross. That means that Christians no longer have to follow it in order to be clean before God; Jesus makes them clean. We also see that neither shellfish or polyester are ever condemned in the NT. In fact, Jesus explicitly states that all foods are clean.
What we do see is homosexuality reaffirmed as sin in the NT, many times, actually. So, the shellfish and polyester argument truly does hold no water, and only goes to show that someone has the capability to a) do a quick (and lazy) google search and/or b) use an argument that they heard from someone else but actually know nothing about.