Nick Lachey is desperately trying to leave his bachelor days behind -- the famous ex-husband just put his $6.8 million post-Jessica Simpson mansion up for sale ... and it is full-on awesome.


The Bel Air home is 5,214 sq. ft. ... has 4 bedrooms ... and it comes with one of the most baller pools we've ever seen.

No word if dumping the property has anything to do with Lachey's recent reunion with former MTV veejay and all-around jaw-dropping smokeshow Vanessa Minnillo -- but if anyone can get a man to sell off his bachelor's paradise, she can.