Would feminism oppose sex-selective abortion to prevent the decline of the female population?

Apparently not in this day and age.  At least they're being consistent, right?  Choose the legal capacity to make choices over the ontological ability to have choices to make.

If I haven't said it before, I've intended to say it a thousand times:  evil is ultimately self-consuming, suicidal.  It is only parasitic on the good and when it has done all the damage it can do to the particular good off of which it feeds, it turns to other goods and extinguishes them, eventually cutting off the branch on which it sits; or as Tolly put it in the Silmarillion:

...in her uttermost famine she devoured herself at last.

Thus do all evils eventually collapse or expend themselves.  The great 20th century dictatorships made great efforts to cull millions of their own people; Germany was saved only by attracting the righteous fury of the Allies trying to rescue Europe from fascism, the Soviet Union was spared perhaps as much by the demise of Old Joe Stalin but was felled by a gross overestimation of the propensity of humans to work for others before themselves, and China perhaps was a wiser student of history but is these days merely taking new bait for the same trap.

So we see it in the Roe effect.  Evil is sterile and cannot propagate itself, not without corrupting another fecund good.  I would speculate that this is why when Jesus touched the unclean, they became clean rather than He unclean.  The beast may raise its head again but here is one more reason why abortion will end:  there will be no one left to support it.