Normally you would try to stick with .com whenever possible, but if the .net was a pretty good deal, it's not the end of the world to build on the .net. Let's face it - Porn.net is better than 99.9999% of adult .com domains and if a serious player in adult owned it and instead considered building a network around a much worse .com, they'd be fucking stupid. The same could be said for the $x,xxx level of domains - if you can get a really nice .net for the money, I say why not? Yea the .com will gain value and will get some of your traffic, but you'll be getting a considerably more elite name than you would in .com for the same money.
Oh, and I second Pradaboy's rant...what you described is Reverse Domain Name Hijacking and is an atrocious practice that should be eradicated. Big corporations are unfortunately sometimes successful with due to the ridiculous legal teams they have and have gone so far as to completely and utterly bully people off names (read up on
the war Nissan has waged against the owner of Nissan.com sometime - Nissan should be fucking ashamed of themselves over that whole mess). You can actually get found for RDNH if you decided to try that tactic. I haven't read up on it enough but at the very least I'm sure you'd be liable for the .com owner's legal fees in defending the UDRP etc.