[Torchwatch]

Speedos commonly have 3-8cm sides, when you go below 3cm the space available at the front is much reduced and for comfort a pouch is built into the swim suit which now becomes a swim bikini.
A pouch is formed simply by cutting and sewing 2 pieces of fabric along a convex curve. The same basic pouch design is used for bikinis, string bikinis and thongs.
They even add pouches onto square cut swim trunks but more for effect than function. The effect is somewhat dominant.

Most speedos are lined, they are intended for use in public pools and are designed to be sporty rather than seductive, the lining smooths over the front of the suit and obscures the contents in the event of white or light fabrics that go rather see through when wet.
On a quiet beach where groups of people tend to be further apart the atmosphere may be sensual or even seductive rather than sporty and you'll get away with an unlined swim suit.