A beam sitting on a post puts a concentrated load directly at the bottom of the post. If the beam after the house settles, has a ton on it, which is a very low estimate considering drywall and furniture (static load), then add some bouncing kids (live load) Then at the the bottom of each post or each end of the beam, is a concentrated load of 1000lb in a 12 1/4sq inch area.

Want a quick lesson in deflection failure? Put a beam under that post. Deflection will become shear if the load is enough.

Get a post on a footer under there. Move the drain if you have to.