This is a tricky one and I can see both sides of this.
In principle you're right: no one should be obligated to deny themselves the opportunity for a happy relationship until their child reaches adulthood.
But, and its a big but, there are so many situations where people move way too fast into new relationships, often involving additional children, and the existing children are forced to shoehorn themselves into whatever situation the parents want, while the parents delude themselves that the children will adapt.
Blended families are not always the disaster they are painted as on here, but they need to be handled extremely slowly and carefully and the needs of the (existing) children should be absolutely paramount. So many people tell themselves children need a "new father" and force the children to accept someone who they haven't chosen and sometimes don't much like and the kids have to put up with it. Or, worse, have to give up a large amount of their life, space and material goods to share with other children who they barely know. It's a high road to resentment and people should do this at their peril.
On the flip side, though, I do think that for children to learn that relationships don't always work out forever doesn't have to a universally bad thing. Not all marriages last forever and for a child to observe a couple (whether the parents or a parent and a stepparent) separating in a kind and civil way with a positive new beginning, doesn't have to be a damaging experience. Learning that there are ways to separate which don't involve hurt and recrimination is probably a useful life lesson if handled carefully (and assuming, critically, that the children aren't left with their financial wellbeing impacted).
We still espouse a model where the nuclear family is considered optimal when in fact around half of us don't live this life. Clearly a string of unsuitable "stepfathers" and undesired siblings is something to avoid at all costs. But for children to learn to manage separation and change without acrimony and to learn that people can survive the end of relationships and grow stronger for it is not necessarily a bad life lesson.