Sex causes preterm labor

Jemma • Hi
So my doctor told me 2 weeks ago... when I was 28 weeks that my cervix was 5.8... which is great with twins... today during my sono... I'm told I'm at a 2.6... and now have to be on bed rest... she asked me about my sexual activity and then tells me.. sex can cause preterm labor.... I'm just like.... well thank you doctor thank you xD