If you really want to invest some time into it, you could look for what you need in Japanese. I personally don't know how to say it "off the bat", but you could just search for key words, e.g. 妻 is 'wife', バレる is "to be exposed", and 中出し is "creampie" (though there are different ways for at least the first two, like 奥さん and 発覚 respectively)
I actually searched a bit, to practice my Japanese as well, and it seems that it's better to use バレる together with the one who finds out (e.g.
this title about being found out by your wife). So, in our case, we need to switch to 夫, "husband". Searching for something like 夫にバレた+中出し+NTR gives us
this series, which might be worth checking out
You can do the same in English, of course, but in my experience, searching in Japanese directly sometimes provides results that you wouldn't get to in English, at least not that easily
The next step would be searching on Japanese AV sites (the official ones might work even better for searching, and then you can find what you found there on another site, for example. It comes down to being creative and specific, thinking like you would if you've made such a movie". For example, you can phrase it using 股, "thigh", 精子, "sperm", 流れ, "stream"; even if this is not how you say "sperm stream [down] the leg", these words might be somewhere in that description
And while I was writing all this, I also remembered about
this one, though it's about mother and son doing it behind the father's back