This is not just a generalisation but incorrect, unless you are basing it on your experiences at Pompey in which case you would be correct. "Some" football fans might be more accurate because I know many people who go to footy and are very civilised. It's an easy comment to make because the behaviour of some reflects badly on the rest (majority). If you believed the media, and clubs continually telling you what they won't tolerate, you would think that football crowds are full of racist, homophobic louts which simply isn't the case. Most of today's racists are at home on their phones and laptops, those that are at games are seldom heard in my experience. Also don't forget there were accusations of racist comments coming from England fans at Edgbaston just last Monday at the cricket.
Having said that, I agree that you are much more likely to be tolerated as the opposition supporter at cricket than at football. The tribal culture at football is more developed for sure.
What does interest me though, is a lot of football fans also go to cricket but seem to conform to the cricket code of conduct when there.