The question to be ask is what is SEX?,What is Love?,with proper definition of these two words you can now say if truely sex is the fire of love in a relationship.
Love that we all look for is within us while must we look for it from some one else.
What is outside can not be shown but what is inside is what is been ask to show.
Imagine if the guy or girl you call your lover is a career of any of these virus (AIDS,HIV etc) and you find out on the process of both of you on the bed will you go on or stop?
Any guy or girl  that have sex with you doing relationship is directly saying YOU ARE NOT THE ONLY ONE HE OR SHE IS DOING IT WITH. Thanks