Fgs! Women wear bikinis which show the exact shape of their breasts! Why can't a man wear speedos!?
Because speedos leave nothing to the imagination and that's his daughter who can see the shape of his dick. That alone should be enough.
But if you need more reasons...
Because bikinis have pads so you don't see the nipples.
Because bikinis don't show the contours of the vulva (and if they do, you can fix it by pulling it out)
Because men do not generally show the contours of their penis in every day life. Women's breast shapes are socially acceptable to be seen since you see their shape with even a modest shirt on.
Because speedos are not socially acceptable attire, while bikinis are. Whether you like it or not, it is more acceptable in society to see the shape of a woman's breasts than the shape of a man's penis.
These are all valid reasons, you may take your pick.
Honestly, just talk to ex quietly about it. It's not so hard to wear swim trunks instead of a speedo to the pool. If a mother was wearing those tiny triangles with a thong bikini bottom, I'd be saying they should cover up a bit for their child too.