I don't see any of the Sith oppose it, unlike the Jedi in some era. Their code is about power and passion, so they just want to release and control it at the same time?
Because crazy, Most sane sith realize restraint = + to sanity and + to sanity keeps you alive longer. family, kids, a lover has that effect
The Sith, by their nature, are about breaking rules. Anything other than general guidelines is just asking for conflict. The same thinking makes marriage unlikely by a Sith. Love is a weakness, something that can both weaken a person's connection to the dark side and provide enemies an easy target. Accepting the constraints of marriage - sticking with someone no matter what, monogamy in the vast majority of cases - is also unusual for a people whose code focuses on breaking chains.
The Sith are probably the kind of people who make ball and chain jokes. And they'd also use actual balls and chains. Missa ab iPhona mea est.
It'd always be incredibly messed up: I loved my partner I married it I killed it It was a weakness I am Sith The notion swanning around for years that you can have a nice, tame Sith is rubbish.
The reason is fairly simple as the Sith Code is based around the simple premise of "Whatever, I do what I want" ala Cartman. Forbidding love is pointless because few Sith actually ever develop the mono-psychosis Darth Bane's order did. The Sith is about breaking "chains" so there's no real "Do nots" in the Sith but only "Dos." Their love tends to be possessive and psychotic, though.
Sith think inwardly only about themselves, therefore the only thing they love and really cherish is their own power
In FOTJ, we see the Lost Tribe's take on love (and Ship's). They see it as something dangerous, but also something that can be turned to Sithly ends. Lady Rhea suggests that desiring something or someone strongly can be used to fuel oneself - but also warns Vestara never to love anyone or anything so much that she cannot bear to lose it. Vestara's father tells her that it's OK to let herself fall in love with Ben - it'll make it easier for her to manipulate him if her own feelings are real. Ship says of love that it is also pain - and that pain makes Sith strong.
You also have to consider, some Sith's lives aren't given in full detail. Some Sith might be married, others might have multiple marriages. They may have been done in secret, like a Vegas marriage. Or they very well could be single. Also to think about it, Sith who don't engage in monogamy either don't fear STDs, or that part of their life does not exist. Yet, I've never read a book of a Sith dying of Aids.
In the Legends continuity, the Sith are rarely 100% pure evil Thanos or Darkseid types. The vast majority are just selfish greedy Jedi. More Lannisters or Lex Luthor. They often have loved ones or associates or dynasties of children. They wouldn't have much of a civilization, really, if they didn't marry or have children. It's a distinction they don't often acknowledge, though.
