This is an interesting post. When you write “Il n’y a jamais qu’une seule explication”, there may be indeed two different meaning. Depending on the context, either you would mean there is always one explanation and one explanation only or, conversely, you would mean that you never have one explanation only.

But in the first sentence, “je ne cherche jamais que le plaisir qu’une chose peut causer aux femmes”, the narrator actually always search for the pleasure that anything can cause to women (the English translation is certainly poor, but it may give you an idea of the meaning of the sentence).

That’s a tricky one. Actually “jamais” can translate to both “never” and “ever“. It’s pretty rare and dated though.

You can find sentences like “A-t-on jamais vu …” meaning “have we/has anyone ever seen …“. You often have to go with your guts and guess which one it is.

In this sentence, “jamais” means “ever“. If he wanted to say the opposite he would have used “seulement” or “uniquement“, putting an emphasis on the “only”.