It has to do with the mind/body dualism still dominant in human psychology.
The body does not lie, whereas the mind, using words, symbolism, can, and often does.
Agape is friendship, kinship of mind - shared ideals, goals, motives, interests, attitudes, awareness.
Here fidelity is called loyalty.
Eros is agape plus the physical aspect.
Sometimes the agape precedes eros, sometimes the reverse, it follows eros - developing after intimate physical contact over a period of time.
This is how arranged marriages remain strong in time - that and external socioeconomic cultural pressures pushing the pair together, when they stray, or begin drifting apart.
Infidelity here is physical, since the body is the main contact between the participants.
In friendship the contact is mental, intellectual...emotional.
The external world is also a factor in agape.
It is in relation to world that friends bond in opposition to, as distinct within it.
Infidelity for a man, in an erotic union is more important than it is for a woman, for obvious reasons.
Infidelity, disloyalty, for a woman, in an intellectual, emotional, union is more important, again, for obvious reasons.
The male needs to guarantee that whatever offspring are produced by the union are his own, and a woman wants to guarantee that whatever offspring are produced by the union are taken care of....that she has a reliable helper.
In the past it was the tribe....a groping of females, as with the lion pride, so the male's agape was not that important.
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