Not sure whether too much sex will stop it happening, but being anxious about is never a good thing. I've heard of couples who have tried unsuccessfully for years, given up the fight and relaxed, and then found themselves pregnant a few months later.
I certainly don't think it's too soon. The right time to start is whenever you're ready. We started trying after seven months and I fell pregnant a couple of months later. I had all the Dutch Aunts' and Uncles' advice - it's too soon, enjoy yourselves whilst you're young (I was 20), you'll regret it later - all that. My eldest is now twenty-seven, the youngest twenty-five. I'm still only in my forties and still young enough to enjoy life and continue to do so for a few years to come. I don't feel I've missed out on anything and my kids certainly don't regard me and their dad as 'old'.