"My boyfriend loves cats."
Yes, I remember 朋友 as two people 朋 holding hands. This indicates friendship in my opinion. Some of these characters have a visual meaning to it that help me, and yes, 男 means boy. I don't have a reasonable answer for that, but I do know that if you learn Korean, 'nam' (남), means boy, which sounds similar to 'nan' (男). However, 'nam' is an adaption to 'nan'.
The origin of 朋 (which looks like a pair of characters for "moon") is somewhat obscure. According to some sources, 朋 is a picture of shell money (such as strings of cowrie shells); according to other sources, 朋 was originally a character for a phoenix (or some other fantastic bird), depicting the bird's wing(s), and came to be used for "friend" because of the character's associated pronunciation.
The 友 part is indeed a combination of the "left hand" radical with the "right hand" radical. The left hand and the right hand cooperate and work together as friends do. If two friends are walking side by side, holding hands, one friend's left hand will indeed be holding the other friend's right hand. Friends often use their hands to greet each other, or when parting (waving, handshakes, or gestures). These are ways to remember that the combination of the left-hand and right-hand radicals mean "friend."
又 is the "right hand" radical. the "left hand" radical is the other, "upper left" portion of 友, the horizontal line intersected by the downward stroke that curves left toward the bottom.
For whatever reason, I can never get that left hand radical to appear, here: every time I try, the app eliminates that radical and everything that follows all the way to the end of the post. Hence, this postscript.
That's right: field over strength. A strong man works in the fields; a man uses his strength to work the fields.
The "knife" character does look a little like the "strength" character, but the difference is that the two strokes do not intersect in "knife," but do intersect in "strength:"
刀 = knife
力 = strength
After all, the only good in plural/singular markers is that you can indicate if there are only one thing or more than one thing. You can't separate between two things or more than two things, which would be nice in case of shoes, for example.
Also, you usually can't apply plural to uncountable objects like air or water even in English, but you can count a glass of water or three glasses of water in both languages. In Chinese the plurality is just irrelevant until you know how many. You can always put emphasis on singularity by adding yi2 ge4 (or whatever the proper measure word may be) if you think it is necessary.
For all Korean speakers, an easy way to remember 'boyfriend' is to remember that the first character sounds like 남 as in boy. The most reasonable reason is probably because 男 originated in China, and 남 is just an adjustment or an adaption to the original word, as I'm guessing. The character 朋 can be visualized as two people holding hands, which is what friends do.