Not knowing why he does it is a problem. That means it's an addiction, or at the very least a compulsion. Phone sex is likely a big turn-on for him (perhaps to the point of a fetish) and if you aren't around for him to call and moan into the receiver to, he'll pay for it. I don't think it's necessarily a gateway to cheating; it's a weird gray area since he's talking to an actual person, but phone sex lines are basically just another version of porn. It's fantasy; he's not going to run off with "Crystal," the phone sex operator who claims to be a 22-year-old coed but is probably a mom of four in Topeka sitting in some skeezy call center. But the fact that he seems to be addicted to calling could be a huge drain on your relationship and on his life in general.

Tell him that while it makes you uncomfortable, you want to help him stop. It must be a huge financial drain, for one thing. To be honest, I didn't even realize phone sex lines even existed anymore, what with the Internet and all. I remember those ads for 1-900 numbers that always played on Comedy Central late at night during the late '90s always seemed like total scams. Then there were the ads for so-called "dating" chat lines that were clearly phone sex. Hey, look, it's Lost star Evangeline Lilly in an old ad for some chat line called "Live Links." If you ever need proof that the people in those ads were actors, here it is.

If he's a decent guy, he probably feels bad about talking dirty to strangers behind your back (and blowing money) and wants to stop. It could be worth his time to talk to a therapist that specializes in addiction. Certainly if you're planning to get married, it is time to wean himself off of his addiction. Be firm, but supportive-- let him know that you can't move forward with him if he's going to be calling phone sex lines, but that you also love him and want to help.

For some people, his actions might be a dealbreaker. But it sounds to me like you guys have a great relationship, and he isn't compensating for something that he's not getting from you. This smacks of a problem to me, and if you love him, I think it's worth getting him some help.