A.T. Robertson Commentary Mark 12:25

"For when they shall rise from the dead, they neither marry, nor are given in marriage; but are as angels in heaven." — Mark 12:25 (ASV)

When they shall rise from the dead (οταν εκ νεκρων αναστωσιν). Second aorist active subjunctive with οταν (οτε plus αν). Matthew 22:30 has it "in the resurrection,"Luke 20:35 "to attain to the resurrection." The Pharisees regarded the future resurrection body as performing marriage functions, as Mohammedans do today. The Pharisees were in error on this point. The Sadducees made this one of their objections to belief in the resurrection body, revealing thus their own ignorance of the true resurrection body and the future life where marriage functions do not exist.

As angels in heaven (ως αγγελο εν τω ουρανω). So Mt 22:30. Luke 20:36 has "equal unto the angels" (ισαγγελο). "Their equality with angels consists in their deliverance from mortality and its consequences" (Swete). The angels are directly created, not procreated.