LEGALLY BLONDE beauty Reese Witherspoon is furious that her ex-hubby, Ryan Phillippe, is getting ready to tie the knot with his sexy young girlfriend – because she doesn’t want the college coed playing mommy to their kids!

“Crash” star Ryan, 39, has been dating 23-year-old Stanford University law student Paulina Slagter for more than two years. And the romance has gotten so serious he’s telling pals he wants to get hitched before his 40th birthday in September.

“Reese knew the day would eventu­ally come when Ryan would remarry, but not to someone so very young,” said a source. “She told Ryan, ‘There’s no way this woman is prepared for a ready-made family.’”

Reese, 37, and Ryan split in 2006 after seven years of marriage. They share custody of their 14-year-old daughter, Ava, and 10-year-old son, Deacon. In 2011, Reese married talent agent Jim Toth, and they now have a 19-month-old son, Tennessee.

Although Reese and Ryan have maintained a civil relationship for the sake of their kids, the actress’ attitude toward Paulina has become a point of contention for the former spouses.

Ryan doesn’t believe Reese has a right to claim any moral high ground after her embarrassing arrest last April for arguing with a police officer in Atlanta while he was busting her hubby for allegedly driving under the influence. “Ryan kept his mouth shut about the incident and didn’t insult her publicly,” the source noted. “Meanwhile, he has every confidence Paulina will be great at helping raise their children.”