OP's question is:
Is love conditional or unconditional? How far must a mate go to keep the relationship going?
should be:
Is seksual attraction between mates conditional or unconditional? How far must a mate go to keep the relationship going?
What that relates is though related, love for a mate and seksual intimate attraction to a mate are not in a lock state. Thus that is not a black and white issue. Even if a mate is still very attractive, there are many interpersonal factors in marriage, especially due to jealousy, argumentative conflicts, nagging, seksual methods, lack of loving non-verbal facial expressions during communication, boring repetition, and interference from many others since many today are exposed to so many other people versus centuries ago, all factors that may distance couples from each other.
In today's telecom world, many people have also become addicted to pornography that may change what a person seksually desires for better or worse whether they choose to believe such will or not. One becomes what one does because of neural plasticity.
At a personal level, I as a lifetime unmarried heterosexual person, am not seksually attracted to women unless I can visually see their body, feminine face, and sense their overall femininity. So am not one of the majority that prefers "light's out". Accordingly am not attracted so in that way to women with overweight, wrinkled, unfit, bodies, or those with less feminine characteristics. But that in no way, means I cannot like and accept such people in other ways.