It has to do with male and female subjective consciousness. The male objective consciousness on sexuality is visual oriented, whereas in the female it is emotional ly oriented. If a male is interested in male or female, it is easy to turn on visually alone as there is no need for emotional involvement. This ease on getting involved sexually translate as promiscuity, it is the stereotype for cis-male both straight or guy, differ only in their object, i.e. male or female. For MTF behaving indecently or promiscuously in public is due to this male consciousness that is still latent. No doubt, some cis-male can behave decently and faithful to a partner, but these are rare cases, the fact is male response to opportunities, if they are more opportunities to meet their objects, and they have the qualities that attract their objects, then opportunity will not allow them to be decent and faithful. In case of MTF with the latent male consciousness who become more attractive to male, then the condition will allow them to be promiscuous.
The female objective consciousness has a filtering mechanism, it seek security as well as emotional feelings, so the object has to be able to fulfill these requirements before she can open herself to the relationship. So female consciousness != male consciousness, and this difference exists as stereotypes of the two groups (male and female consciousness whether it is in the cis-male, gay or MTF). Lesbians have female-consciousness (thus more emotionally oriented and faithful to their partner), even FTM carry this latent tendencies (but successful FTM may adapt to the male culture and completely changed their subjective consciousness).
Whereas the female subjective consciousness is highly self-conscious, i.e. care about gracefulness, decency and morality in self behaviour, speech, looks, etc. Thus this consciousness expresses motherhood in the female, it opposed to behaviour can be potentially attract harm to their children through indecent, and promiscuous behaviour. Successful MTF should be able to adapt to this female consciousness which prepared them to be good mothers (though they may not be one naturally).