Genocide is the intentional destruction of a people[a] in whole or in part. In 1948, the United Nations Genocide Convention defined genocide as any of five “acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group.” These five acts were: killing members of the group, causing them serious bodily or mental harm, imposing living conditions intended to destroy the group, preventing births, and forcibly transferring children out of the group. Victims are targeted because of their real or perceived membership of a group, not randomly.[1][2]

Spreading an ideology according to which one shouldn’t have kids, thus preventing births, would fall into this definition.

Even with this extended definition, your argument fails the most important criteria for genocide wtih the UN definition which is:

intent to destroy, in whole or in part, a national, ethnical, racial or religious group.

And it also fails to mention that the argument being made is voluntary and so it wouldn't fall under the act of :

preventing births