A lot depends on what your current and hoped for future relationship will be with the married couple.
We had a similar situation of a nephew who got married. They had a small wedding as both of their parents disapproved on racial grounds. SIL was so upset she never mentioned it to us until we were at a niece's wedding a couple of months later. The niece (nephew's sister) essentially forced her parents to invite nephew and wife to her wedding.
Our standard gift for a niece/nephew marriage is $1,000, and this nephew was never a problem for us, always polite, thanked iust in the past when we sent birthday/Christmas gifts, etc. So we sent him and his wife a congratulations card with a check for $1,000. TO say they were overwhelmed was an understatement. We were able to give them solicited personal finance advice, as one of their first questions when they called to thank us was "we don't know what to do with this money!"
We have also given cash gifts to several of our kids friends who grew up with them, particularly the ones who overcame difficult circumstances. For example, last year with the pandemic one woman got married, she had moved far away and we were not invited. The wedding was limited due to the pandemic, and we would not have been invited if it had not been, but this woman had been a great kid growing up and was always friendly and kind to us, so we sent $500. I admit I enjoy making folks cry for good reasons
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Our attitude is, many of the young people we know who are starting out in married life these days have enough things to deal with. We are not going to let the lack of an invite to a wedding dictate whether or not we give them a gift.