Polar does have special sports bras that the HRM strap fits into, so the sensor part of the strap is the only part that touches your skin... the rest of the strap lies against the outside of the bra.
I ordered this bra about a year ago because I don't like the feeling of the HRM strap against my skin, but it was extremely tight and uncomfortable so I returned it. The Web site warned that they run small, but they must run VERY small because I'm normally a M but the L felt very tight and constricting.
If you do order the Polar bras, order a much bigger size than you normally would. Or order a couple different sizes so you can be sure you have the right size... their sizing is just goofy.