Vanilla sex, or sometimes just vanilla, is a term used by some people to describe either sexual acts or sexual conventions that are considered social appropriate and 'normal.' Calling someone vanilla, or someone's sex life vanilla, suggests that what they do is plain, ordinary, and usually has a negative connotation of boring or bland.
The use of the word vanilla to describe sex dates back at least to the early 1970s, when it was used in sexual sub-cultures do distinguish non-SM gay bars from SM bars. In that context vanilla sex is used in opposition to the label kinky sex.
Of course there is often a difference between how people present their sex lives in public and what their sexual thoughts, feelings, and activities are in private. For this reason, while the term may help some people distinguish themselves, unless it is being used by someone to describe themselves, it remains more of a value judgement than clear description of a person or their behaviors.
Oxford English Dictionary. Second edition, 1989; online version June 2011. Earlier version first published in New English Dictionary, 1901.