Yes, it is always immoral and is not focused on enough by Evangelicals:
There is no circumstance in which it is acceptable for people to have sex outside of the confines of a Biblical marriage. The "religious right" is right to condemn homosexual sex as wrong, but it is not strong enough about condemning all heterosexual sex between unmarried men and women- this includes everything from one-night-stands to long-term relationships that have not resulted in marriage.
I don't want to get off topic, but I wonder if that's because the latter hits closer to home for many of us- how many evangelicals have sinned by having sex in a long-term heterosexual relationship, but before marriage? Even the best Christians sin, and we cannot simply condemn those that are foreign to us (I don't think I regularly interact with a single non-straight person, for instance) while ignoring those that impact our communities more (I know plenty of Christians who have had pre-marital sex and presume many that I don't know about have as well).
Also, just because something is a sin and a person is a sinner does not give us permission to treat anybody with disrespect. Every person on this Earth sins, which is why Jesus died for our sins. We must respect sinners as people, but we also must do our best to not respect their sins.