David Bowie's response to the matter (as reported in US magazine in 1995) was succinct:

"About 15 or 16 years ago, I really got pretty tired of fending off questions about what I used to do with my [penis] in the early seventies. My suggestion for people with prurient interests is to go through the 30 or 40 bios on me and pick out the rumour of their choice."

Bowie's wife Angela DID find them asleep in bed together. And they were naked. But whether or not "anything" sexual happened has been both claimed and denied at various times by Angela herself.

There's a long, long series of versions of this Urban Legend at:
http://www.snopes.com/music/artists/bowie.htm

What does it matter? (Stupid questions get stupid answers.)

That is not true mick jagger never slept with another man mick jagger is not gay he never slept with david bowie

Do you have a bud, like a man who is your best friend, good friend, hang out all the time, a bud comes over and passes out on your bed had a few the night before, and you are drunk to, you can't wake him up but your shirts off and buddies is passed out drunk in your king sized bed. Well I'm not moving off my bed and I can't move Mick, but I'am tired drunk, so bowie's wife walks in with the kids and is pissed off at bowie and thinks of away to spite the two of them, she says I caught them in bed together well every one knows that jagger, and bowie's kids play together now who is going to really believe these spite ful accusations from gal who is in need of attention alot of attention. so there it is if jagger is gay, who fathered the 8 childeren of his. And I don't belive Iman would agree.Do you?

They had a gay old time together

Bowie said maybe it did happen, maybe it didn't, but his ex-wife CLAIMS she saw them in bed together. But do keep in mind that she stated this AFTER the divorced.