My first was a female Australian Shepherd in season, when I was 7 years old.
I had seen male dogs chew holes through heavy duty chain link to breed females, and I just had to know what was so good.
Thankfully it was only a matter of a couple of weeks before a suitable bitch was ready for me, one of our stock working dogs that lived in one of many outdoor pens. I snuck out one night and spent the night with her, going twice. Blue was amazing! Hooked for life.
My first male wasn't until I was 14 years old... an Australian Kelpie stud. Hooked.
I still prefer females 3:1 over males.
My human wife and I had almost 9 good years together before we separated, so I have had a lot of human experience as well, though at this point I would most likely not bother with a sexual relationship with any women again. Friends, sure, but not anything like a traditional relationship with a woman.