Rashi on Leviticus 20:20/רש"י על ויקרא כ׳:כ׳

1אשר ישכב את דדתו AND IF A MAN SHALL LIE WITH HIS AUNT… [THEY SHALL DIE CHILDLESS] — This verse is intended to teach that the "excision" mentioned above (18:29, where "all these abominations includes the union prohibited in v. 14 of that chapter) consists in their (the offenders’) passing from this world childless.

2ערירים — Translate this as the Targum renders it: בלא ולד, childless. Similar is, (Genesis 15:2) "and I will pass away childless (ערירי)”. If he has children he will bury them (i. e. they will die during his life-time); if he has no children when this sin is committed he will die childless (he will beget no children afterwards). That is why Scripture varies the expression in these two verses (this and the following) here stating ערירים ימתו, "they shall die childless", and there ערירים יהיו, "they shall be childless". The expression ערירים ימתו implies: if he has any children at the time he commits the sin, he shall have none remaining when he dies because he will bury them during his lifetime; ערירים יהיו on the other hand, implies: if he has no children at the time when he commits the sin, he shall remain all his life just as he was then (Sifra, Kedoshim, Chapter 12 7; Yevamot 55a).