No. If their marriage after your 18th birthday, you cannot file for your stepfather for US immigration purpose.Yes, he is still legally your father, but just not the father that can get US immigration benefit from you.
Think of 2 senarios: 1. You married a US citizen but you get GC from EB based category. Your marriage is none of USCIS's business. 2. You married a US citizen and you want to get GC based on being a US citizen's spouse. You have to prove the mariage is true to USCIS. They have every business to exam your marriage.