I don't think there's a difference between Sid and Roshni's individual plans.
It's just the fact that we see him as a guy doing it, that we get more pissed. As a trope, in general.
For example, Khushi did it for a bit in IPKKND to find Arnav from Shyam. I'm sure many other shows have had the bahu do that. But with Sid, being a guy, even if he's seducing for the greater good (well from his pov and plan), we get a bit more disgusted as an audience.
I don't think he failed as a husband. Yes, ROshni probably will get hurt but for the time-being, the plan is more likely to be successful if she doesn't know about it.