Florians!

You wonder what this is? It's A Florian, a plant-based alien from another galaxy. Whenever they have sex, be it with a human or another life form of some sort, they secrete a yellowish fluid similar to honey of plant nectar on Earth. As an added bonus, they can't get pregnant from a human because although their reproductive organs are similar to ours, they function very much like Earth plants!

Here's a humanized version, for those who doesn't want to have sex with an alien.