We are a mid 30s couple from Australia and we did a river cruise in Russia last year which was absolutely fabulous. It is a totally different experience to an ocean cruise and because the ships are so narrow (they need to fit into the locks etc necessitated by river cruising) the cabins are a little cramped compared with ocean going vessels but the overall cruise experience was great. On our cruise there were at least three other gay couples whom we got to know.
The average age of passengers on our cruise was probably early fifties and the absence of any children was notable, but I think because of the destination (Russia), it seemed to attract a more well travelled and educated group. At no time did my partner were my partner and I made to feel awkward by either staff or crew because of our sexuality an by the end of the cruise we were just referred to as "the boys". (Mental Note: if you want to be made to feel young travel with folk older than yourself)
We travelled on Viking River Cruises which we found to very good value particularly as all the shore excursions were included in the price.