is it wrong to withhold sex from my boyfriend?

Before you answer, actually read my reason why please. We've been together for almost four years, no kids but we do live together. I don't want to force him into engagement by holding sex over him, but I feel like he's prolonging engagement because we do already live together and he's getting sex. The old saying, "why buy the cow if you can get the milk for free." Is it wrong if I tell him that I want to abstain from sex until our relationship moves forward because I feel as if this is the reason? He talks about marriage and whatnot all of the time. Just never actually makes the move.