You had asked for a source against mixed swimming. Don’t forget that swimming attire at the time these teshuvos were written was far more “modest” than currently. Also note that the many who wrote against it (see ???? ??”? who collected many teshuvos – http://hebrewbooks.org/pdfpager.aspx?req=41167&st=&pgnum=162&hilite=) didn’t seem to feel that they were not entitled to an opinion because they didn’t have “personal experience” (to address flatbusher).
In terms of the general issur, he groups them together: ???? ?????? ?? ?? ????? ??????, ???? ????? ??? ????? ???? ?????? ????”? ???”? ??? ???? ??? ?’ (???? ???), ?????? ??? ??? ????????? ????”? ???”? ????? ??? (??? ??? ???? ???? ??), ?????? ??? ??? ???? ????”? ???”? ??? ??? ??? ?’ (???? ???).
Second, he argues on the application of ???? ??????, but the ??”? didn’t discuss how likely the necessary condition for ???? is: ??? ???? ???? ?????.
For this, we can apply the various ????? ??”? quoted in these teshuvos, and which R’ Moshe doesn’t address. It is quite obvious inherently, and from our ?????? in having a ????? for dancing that seeing vigorous movement is ???? ?????, and for obvious reasons, ??? ??? ???? ??????.
R’ Moshe addressed ????? (and even then, under such limited circumstances as to render an actual heter nearly impossible) but didn’t address “getting in shape”, and there are numerous options, even with no “kosher gym” in Flatbush, even for ????? and ?? ??? for getting and staying in shape, there are numerous options, rendering it ???? ???? ??????, and ???? even in the rare circumstance of someone who can fulfill the other condition. It’s fair to say that we can apply to someone claiming it’s okay for him, that which is quoted in the name of (????? ??? ???? ????’? ???? ???? ??? ?? (???? ?’ ??? ? regarding the overall issue (different case):
????? ?????? ???? ?????? ????? ????? ??? ??????? ??? ?????? ??