OK, if the theory that the couple remained childless because Paul's children fulfilled Sergei's desire for fatherhood is not so valid, given the fact that they didn't appear in the picture for seven years
after their marriage, then there are two other possibilities, same as those mentioned before. The most straightforward is that one or both of them had fertility problems, something which we may never be able to prove. The second, more theoretical "amateur psychologist" one is that, if the cause of their childlessness
wasn't physical, then it could've been due to Sergei not having relations with Ella (due to his apparent natural sexual aversion to women or due to the rather strange reason given by G.D. Alexander that she was just too precious to touch). Therefore, they may have resigned themselves to being childless (due to his selfish reasons) until along came Paul's children. And then suddenly he had the children he wanted but may have been unwilling to conceive, so they continued without children of their own.
I agree that Sergei was seemingly not very capable of being a loving and intimate partner outside of the marriage bed, much less in it. He was still expected to marry and maintain the facade of a normal marriage, but within that marriage's exterior shell things were very different. I don't know much about Sergei, and thus I wonder if he ever had a female lover(s) before he got married, if he showed any sexual interest in women at all, in short - if he was turned off by all women or just by the one he had to marry? Because it would seem odd to me that, if he
did have prior lovers, he would have had relations with women while he was single but then didn't want to with his own wife (then again, those other relations could just have been something he didn't really enjoy and what it took for him to discover he was gay). Disclaimer: Remember, this is all my conjecture!