This question already has an answer here:
He wasn't human, nor did he feel any affection towards anyone. Now that we know his body was human enough for him to reproduce, why did he do it? If he just wanted to carry his bloodline forward, why then and not earlier? If it was a mistake, why was he fooling around in the first place? Bellatrix was a married woman, so I can't imagine him wanting to indulge in shenanigans with her when he could've easily Imperiused anyone in to being with him, because he didn't necessarily require a partner that loved him. The above applies even if he was just feeling curious or missed the sexual activity.