I agree being a stepchild is shit. I was one with both a stepfather who literally became ‘Dad’ overnight and a stepmother who I never met (nor did I see my father again for over 30 years) as she wanted my father for her children.
I agree that the shitness was all down to my parents though. They should have put the children they already had first, made sure they chose new partners who would accept and have affection for their children and not created situations where their children were neglected and felt rejected or left out. They also should have considered the impact that additional children would have on their existing children.
If I hadn’t had the experiences I had as a child, I probably would have divorced DH long ago. We ‘fell out of love’, argued a lot, that was in the midst of the stress of raising 4 kids though. I swore they would never have a stranger living with them, feeling on eggshells in their own home, not being comfortable to be themselves, not feeling like a complete family etc. I was also worried that DH might swan off to a new family and forget about our DC like my Dad did and I couldn’t risk that. So we stayed together with lots of downs but some ups too! Now I’m very glad we did, DC are older, our relationship has a 2nd wind and we’re very happy.
If we ever did split up though, I would never inflict a stepfather on my DC. I might have boyfriends who certainly wouldn’t be living with us and even a new husband when DC have all left home. My DCs well-being is more important than my love life.
Funnily enough, my ‘stepsister’ praises my father as being a great stepfather to her and adores him 
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