Have I understood the joke 男：我一直就很爱你啊！ 女：那不直的时候呢？ correctly? I found this joke on bliubliu.com:

那不直的时候呢 (爱情笑话）
Nà bù zhí de shíhòu ne (àiqíng xiàohuà)
男：我一直就很爱你啊！
Nán: Wǒ yīzhí jiù hěn ài nǐ a!
女：那不直的时候呢？
Nǚ: Nà bù zhí de shíhòu ne?

I want to make sure I've understood the joke correctly.
The man says 我一直就很爱你 meaning I have always loved you (I think).
However, the woman thinks the man is using the 一。。。就。。。 grammar construction, and consequently thinks that the man means that when 直 occurs, his love follows.  This is why she asks about when 直 is not happening.
As far as I can tell, it doesn't make sense to talk about 直 happening or not happening, and the woman's question is nonsense.  But maybe I'm missing something.
Question: Have I understood this joke correctly?
直 could mean heterosexual(informal).

This comes directly from "straight" in English.