We both do this. It's fun and it marks my territory, as someone else just said. It's also very sexy and turns us both on. MMMM!
I have a very strong sense of smell and taste, I can smell things miles away, I think. The smells of sex with two clean people is amazing! Those aromas are there and are made from pheromones to help us get turned on.
If you really investigate it, your (clean) mouth and your sex juices smell and even taste similar based on your body chemistry, what you eat, how turned on you are, whether you have had an orgasm yet or not yet, and other factors. Animals smell and lick each others genitals for good reason! They know what they are doing.