Sex and love are two quite separate things. Sex is physical and usually interactive, whilst love is an emotional state of mind - although it can be described as a chemical reaction. One has sex with one's partner ... or with 'friends with benefits' ... or with 'fuck buddies' ... or with those who sell sex ... and so on and so forth. Of course it is natural to 'love' one that you are having sex with. Equally you can describe the feelings that you have for other people such as love your children, relatives, friends, people in society that one admires in an entirely non-sexual way; people also 'love' certain foods, places, times, fashions, and so an and so forth. So it should come as no surprise that love and sex induce different physical reactions in the brain.