Hi Sam,
Sorry to hear you are having troubles.
Personally I think that it is no one's 'fault', but I can see that a pattern has evolved and leads to the same outcome day after day.
From a woman's perspective... libido varies considerably from person to person, and for a woman there are many environmental factors that will have an effect.
You mention that your wife works and you have children also - these 2 factors will definitely impact a sex drive, not to mention anything else going on in your lives (fights about sex don't help either!).
The OCP (the pill) is a bad idea for contraception - the pill prevents ovulation (which is what you want with a contraceptive), but a side effect of this is that the hormones released at the time of ovulation (the 'sex drive' hormones) are suppressed. The end result is a reduced libido which, with long term use, can lead to a permanent libido loss. This being the case, perhaps you need to explore other contraceptive options.
Now, a point that probably hasn't been raised (sorry, have skipped over the last couple of entries...) - if you force sex on someone that has no desire to have sex, the result can be horrendous. Forcing a person does not necessarily mean physically forcing - it can also be coercing, or guilting them into it. Sex is pretty much the most intimate act a person can do, and if you make them do that, they will resent you. What you really want to achieve is having your wife WANT to have sex with you.
Please ignore all the advice listed above suggesting that you make your wife jealous, or threaten to find sex elsewhere... this will NOT achieve what you want, it will make your wife resent you more. You mention that in Indian culture that women are not meant to enjoy sex... this probably has some impact, I know that for other people the old 'Catholic guilt' has an impact, as would other cultural and religious influences.
What I suggest is that you seek professional advice - a recommended psychotherapist in HK as per asiaxpat (I have not personally used this service, but the website looks good:
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Best wishes.