Why is it that whenever I tell one of my Thai lady friends that I have a girlfriend, their next question is when are you getting married? (My reply is always "never" by the way.) How do they see me having a girlfriend as me getting married?

Interesting.

If that "never" comes to your g/f's ears......she won't be staying with you for much longer.

Stop telling your ladyfriends you have a girlfriend.

they don't want the two of you live in sin...

Basically the Thai lady's way of thinking is you will be getting married or you "BOOM BOOM" your G/F for free, it's connected with sin-sod

Isn't that what people everywhere ask? But if the truth is that she's really not your GF, but just a convenient and temporary f***-buddy (unbeknownst to her I'm sure), then that's what you should tell your friends. I'm pretty sure the "when getting married" questions will stop.