Am I right in believing that immorality, that is sex outside the context of love, is wrong, not because it contravenes some priestly law which makes it a ‘sin’ but because it is the spawning ground of AIDS and many other sex related diseases?

First define love. There are women who claim to have been in love with one night stands and most people would include this in immorality. However, let us assume that in all probability what is meant in the question is true love, which calls for the maximum commitment and responsibility. Sex outside this is wrong for several valid reasons, but let us deal only with the question. It is true that nearly all sex transmissible diseases have been generated through the abuse or prostitution of sex, but this does not mean that some such diseases are necessarily the outcome of casual sex. On the contrary, the odds are in favour of not catching any disease, but the possibility is always there and the odds are lessening all the time as the climate of immorality is increasingly fostered. Sex outside the context of love is wrong, not simply because it was in this climate that most sexual diseases were spawned, but because it manifests a state of mind, an attitude, an acceptance, which condones and fosters that climate. Couples free of any sexual disease, who indiscriminately hop into bed are neither generating nor transmitting any sexual disease, but they are promoting and fostering the communal state of mind which condones casual sex contacts which may be transmitters of diseases.