One way tickets are like the big red flag in the airline industry... I don't know where this perception came from, but I do think the vast majority of terrorists only book one way tickets. Pre-9/11, I had to book a few one way tickets and boy, did I get the third degree. Now everyone gets it, but it was remarkable in that day and age.
Your open-jaw (which is what the airlines call a layover, where I am assuming you deplaned in Paris and left the airport) would have appeared on the surface like a one way ticket without the itnerary. Again, I don't know if you left via Heathrow, but that has been pretty typical experience in my estimation. Gatwick, I tend not to get as much interrogation.
Whether it is logical or not, there have been apparently a lot of [url:http://www.thestar.com/World/Columnist/article/552561]fears[/url] around Al Qaeda using females to further their agenda, so I am sure the risk assessment folks at the airlines have decided that the hired goons should try to ascertain the relationship of any males and females travelling together.