I have personally experienced how chatting about sex can lead to more. But that's mainly because the people involved already liked each other a lot, and worked together (opportunity). The reason it started at all, is because the girl wasn't getting enough out of her previous relationship (reason).
If you two have a good relationship, including a healthy sex life, the sex chat shouldn't be a big deal. But I have my doubts about his reasons. As others have said: it's not porn, it's much more like real sex.
You should probably ask why he feels the need to sex chat with other women. IMO, flirting with other women should (at the very least) boost his ego. The flirting itself can be fun too (the hunt, the chase, etc.). Maybe he misses that bit; that might be solved with a bit of roleplaying.
But maybe they "do" things with him that you don't, which can be a sign of problems ahead.