What on Earth, there are no day time night time rules. Just get on with your life with whoever you want no one will stop you or question you unless you do something to offend the culture here. It's important to bear in mind that you re a guest in Qatar so the least you can do is respect their culture even if it may seem 'strict' to you. I think if you show no public display of affection and you'll be okay.

Hope that helped!