Is it a normal cat behavior?

We have adopted a male cat last August and have kept him indoors for a long time. He used to sleep cuddling and snuggling beside me, or my sister... I noticed though that he's not as sweet as he was when we first took him in. He doesn't sleep on the bed anymore - instead he'd find a corner and just sleep there... Also, 2 stray cats come by outside the apartment almost everyday and make that very annoying noise... The next thing we'd notice is our cat's gone out and with them. I want to know if that's a normal "cat" thing catching up with new friends and meeting them outside when they come around? I think my cat has found new pals and has lost interest with the humans... :(