Sex on the side

I work in another state away from my family and at 25 single working at a mill, I was boarding in a house with 2 sisters in their fifties. They were widows and lived together and renting out a room to make extra money. I'm quiet by nature and not the woman chasing type guy. Too many failures and so I had given up, not that I was anything special in looks or build. The sisters were okay looking for their age, a little overweight maybe. After several months I was helping them where needed around their home. On a rainy Saturday afternoon, the younger sister was out at something or another and it was just me and the older sister. I was in my room reading (porn mag) and she came in and straight out asked me for sex. Shocked at first, mainly because it was my first time being asked such a thing but recovered enough to agree. We had sex twice and had no expectations of a repeat anytime soon. But that night the younger sister asked for the same as her older sister and I again agreed. That was the start of a strange sexual relationship, one or the other would request sex with me, never both together. They never had sex with me if I asked them, only when they asked. That has been the pattern ever since, still no girlfriend, but regular sex with one or the other and I've no plans to find my own place either. Why would I, cooked meals, laundry done, and sex provided, I would be nuts to move and I am not nuts.