harry potter - Is there any source saying Voldemort and Bellatrix had affairs?

A friend of mine told me that Bellatrix is Voldemort's girl.

I argued that I didn't agree (because Voldemort never valued love) but he countered with the argument that even evil super-villains may feel the need to indulge their baser urges.

Are there any canon sources saying if Voldemort ever had a sexual affair with Bellatrix?

Answer

Yes (probably).

In Harry Potter and the Cursed Child, we learn that Delphi was (probably) product of Voldemort and Bellatrix Lestrange, having been told so by Rodolphus Lestrange.

As many of you know, a body has been found at Hogwarts. His name was Craig Bowker. He was a good boy. We have no firm information who was responsible for the act but yesterday we searched St. Oswald’s. A room there revealed two things — one, a prophecy that promised — the return of darkness — two, written on the ceiling, a proclamation — that the Dark Lord had a — that Voldemort had a child. The news reverberates around the room.

We don’t know the full details. We’re only just investigating — questioning those with a Death Eater connection . . . And as yet no record has been found either of the child or of the prophecy — but, it does look like there’s some truth to it. This child was kept hidden from the wizarding world, and now she’s — well now she’s . . .

later

DELPHI: I am from the future. The child of Bellatrix Lestrange and you. I was born in Malfoy Manor before the Battle of Hogwarts. A battle you are going to lose. I have come to save you.

HARRY/VOLDEMORT turns. She meets his eyes.

It was Rodolphus Lestrange, Bellatrix’s loyal husband, who on return from Azkaban told me who I was and revealed the prophecy he thought I was destined to fulfill. I am your daughter, sir.

Whether this is true (or not) remains to be seen, but this certainly meets the criteria for "highly-regarded rumours".