(Okay, so I haven't read all of the way through, but I'd like to offer my two cents now and hope it makes sense anyway)
Let's go game-by-game:
In the Sims 1, sims got abducted without any impregnation.
In the Sims 2, sims got abducted when you used a specific type of telescope in a particular range of time. The chance of getting abducted without any aspiration awards is TINY - without "Summon Aliens" used to kickstart your sim's life of alien contact, I've never had it happen outside of scripted events. Furthermore, the sims who can have the Summon Aliens reward, knowledge sims, are the most likely to want to purchase and autonomously use the correct telescope. (Fortune sims I believe want to buy it, but they just want to buy it for the sake of having something expensive). However, an important point with sims 2, is that the sims most likely to have the conditions set up to be abducted (knowledge sims) get a positive memory! of both the abduction and the birth of their alien, implying it was all wanted (it usually is!). For anecdote's sake, I have a knowledge sim who (I wish I was making this up) keeps going out into the middle of the night to stargaze while I'm busy micromanaging his children to get abducted again! It's such a regular occurrence that when he gets spit out into the street the whole family, half of whom are aliens, just gather around and laugh at him. And he is racking up the reward points because those abductions make him so happy~
Can non-knowledge sims be abducted? Yes. Are the circumstances so specific and difficult to achieve for even knowledge sims that it's easy enough to avoid? Yes!
Sims 3 is where my problem lay with alien abduction. It is random, no telescope necessary, and I've had it happen so. many. times. to my hapless male sims that I can't stand it. THIS is where non-consensual alien abduction and impregnation was a problem. I couldn't prevent it, just had to watch in pain as my male sim got slurped up into the sky with my fingers crossed he wasn't experiencing weight gain.
As I understand it, Sims 4 follows the Sims 2 model (...more so than 3, in terms of requirements. I still can't stand the ~anomalies~ replacing the telescope), and so I think that the amount of work and chance involved is enough to require at least the player's consent, unless you have the funkiest luck in the world.
tl:dr - don't nerf them, toggle them!
