You don't *have* to prove you're getting married - but you *will* be asked about your wedding plans, especially if you don't include any documentation showing that you've started making said wedding plans.
I would strongly advise you not to apply for a fiancee visa on false pretenses (in other words, applying for a visa, getting approved, and moving over with no intentions to get married within the six month period stipulated). If you don't get married and convert your visa to FLR before the fiancee visa expires, you'll either have to leave the country at the end of six months or risk overstaying your visa - and if you're planning on living in the UK at some point, the fact that you didn't meet the terms of the fiancee visa could cause trouble with any future visa applications, especially if you wound up overstaying and/or they find out that you provided false information on a previous visa application.