Sometimes lines like "You didn't think I was serious did you" are when people regret what they said earlyer because they are too embaressed now. When you suprise people (by asking about the shoping again) they have to think quickly, and sometimes their rushed reply can sound a little harsh and defencive.
If you think that marrage is a possibility, then perhaps he actualy was thinking about engagement ring shopping, but was to shy to admit it when you asked about it later on.

Perhaps he is seriousoly thinking about it.
Perhaps he is a little curious.
Or perhaps he just did it for some kinda joke.

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