For me it was my Dad.
My Mom was twenty years older than my Dad when they married. She was 44 and he was 24. She died of cancer when I was five so it was just me and my Dad from then on.
I was seven when it started. He had insisted on giving me a bath every night despite my telling him I could do it myself. I could of course. Most five year old boys are able to do that. But he insisted that it was better to have someone do it for you. Nothing out of the ordinary happened until two years later when I was seven. Just out of the blue he started to spend a lot more time washing my private parts. Within a few weeks it escalated to him sucking my dick and licking my balls and asshole. Then he merely used the bath as a means of cleaning me in preparation for bed time which consisted of him taking me to his bed and licking me all over. It felt amazing getting licked like that. I became concious of being arroused sexually around age nine and became interested in him sexually around age 11. That's when I asked if I could suck his cock like he had been doing for the previous four years. He said O.K. but to not do it unless I really wanted to. I really wanted to. I had been having dreams of him fucking my mouth for the last year or so. I just had to do it. I'm glad he let me. He taught me all sorts of advanced cock sucking techniques. He said I was the best cock sucker he ever met. I was very proud of my ability to make him squirt his sperm down my throat. When I was 12 he finally honored my request to fuck my ass. I know not many 12 year old boys would do that but when your dad licks your ass for a few hours every day for five straight year you start to crave something more. He very gradually stretched my ass out over the next few months using first very small butt plugs then gradually increasing size until he got to one as big as his massive cock. Finally he fucked my ass. It was way better than I imagined. We fell asleep that night with his cock up my ass and he didn't take it out until he had to piss in the morning which was fun because it was hard from having to piss. I almost asked him to piss up my ass but I didn't. He didn't do that for the first time until I was 14.
I'm now 39 and Dad is 63 and I am still his grateful slave faggot. I only regret that he didn't start when I was younger.