We can help you, but you gotta help us help you.
Any more specifications to work with?
Does it include animal life? Is it xenobiology?
Is it only
about humans or humanoids?
The latter rules out “Love Is the Plan, The Plan is Death”.
“son impregnating his mother”
That wording suggests
humans and humanoids – as opposed to animal offspring names like foal, pup, chick, whelps, and so on. But there isn’t much certainty to go on in the question.
If we can narrow it down to human-like people, that rules out a lot of search items.
“where male were also female”
“a collection of short stories”
*Rules out the nongendered novels about the “kemmer” process by Leguin. And collections of her short stories that aren't exclusively that theme.
*Rules out single stories like “All You Zombies” unless it’s in a collection of Heinlein’s similar stories.
*Rules out a Dean Koontz novel about a person who impregnated herself to have her son.
* Rules out Sturgeon collections. He wrote about same-gender couples, and incest, and who knows what else, but it wasn’t really alternative procreation. Many collections of his stories, but (iirc) not one on any single theme.
“explored different strange ways of procreating”
Is the main topic procreation? Is it procreation only? Or sexuality without reproducing? Romance? Is egg-laying involved? Parthenogenesis? Budding? Three-gender triads “melting” to produce an offspring? Interspecies hybrids? Changing through the life cycle from plant to animal?
Knowing these would help include as possible or rule out a story collection.
"Procreation-only" rules out stories about alien sexuality without procreation. It rules out attraction, romances, nonreproductive sex, marriages, sociology of couples (or triples or singles), etc..
If you rule out animals, is it human only?
"Old Folks At Home" by Michael Bishop
is about, well, an “old folks home” which has “marriages” of five or seven people. Never an even number, so that people don’t start to form pairs. All of the women and all of the men in a marriage rotate bed partners every night.
(I wrote him and told him it should be in more anthologies and much more well-known.)
Or is it human-alien?
“Bits” by Naomi Kritzer. Which involves silicone toys to match Earth human and alien… bits.
The subject already has more extensive coverage in "Sex and sexuality in speculative fiction"
The page has more novels and single short stories named that you can rule out.
It describes a collection, "Eight Worlds" series by John Varley
. However it might be too far-ranging; maybe it only includes some of these themes, rather than being about
You might see something you recognize.