If a Kohen who is not married as intimate relations with outside of wedlock, does he still remain a true Kohen with regards to aliyot and all matters of Kehunah? What are the sources?
Yes, a Kohen who has relations outside of wedlock does not become in any way disqualified from Kehunah.
A Kohen receives his elevated status by birthright. The manner in which a Kohen loses this status is if his birth came about through a forbidden relationship of his father (such as if his mother was a divorcee before his father married her; a relationship out of wedlock does not disqualify children).
Once born a qualified Kohen, he is not disqualified by transgression.