I'm looking for a slang word or idiom for someone who insists on intruding their presence on two others who would rather be alone

This person usually pretends not to see that they are unwelcome at the moment, but it may be that they just don't notice it.

Depending on the circumstances, one of the two persons (typically lovers) might even say: "two is company, three's a crowd."

Edit - this question is not about sex. It's about two persons who want to be alone for any reason (because they are sweethearts, because what they want to talk about is confidential, etc.)

Solution 1:

The classic idiom in English is "third wheel". This can be used both against both parties (the couple and the third party). For example:

The couple might say: "He is such a third wheel. I wish he'd stop trying to hang out with us."

The third party might say: "They make me feel like such a third wheel. I don't understand why they invite me along when they clearly don't want to hang out with me."

A more pejorative (and much more vulgar) term might be "cockblock". The implication here is that the intruder is preventing the couple from "getting it on" (having sex) by blocking the male's cock (penis).

"Tommy is such a cockblock. He needs to get a life."

However, cockblock isn't necessarily only about stopping sex. In fact, cockblock has been expanded to include any individual that stops something that people want to do, especially in online gaming:

"Tommy is such a cockblock. I almost got to that extra-ammo pickup, but he killed me before I could. He needs to get a life."

Solution 2:

I have always known such a person as a gooseberry (UK). A quick search finds lots of references but none seem to know where the expression comes from.

Solution 3:

Also the fifth wheel :

• Fig. an unwelcome or extra person. Bill always begs to come on camping trips with us, but really, he's a fifth wheel.

• (idiomatic) Anything superfluous or unnecessary. As the third person along on their date, I felt like a fifth wheel. I felt like a fifth wheel when they started giggling and making out during dinner.

(From TFD)

Solution 4:

Someone who intrudes on others without realizing that he's doing so might be said to be oblivious to social cues.