One way ticket to India

No. But have a reason. Don't leave it open ended and if you are on a tourist visa, don't disclose your marriage plans.

Ok thanks..

What would be a good reason for having just a one way ticket that wouldn’t set off further questions?

US citizen ....not sure of how long I’ll be staying( I’ve planned on a year).

Unless I’m mistaken the visas for US citizens means six months stay per calendar year. So this would involve a visa run.

Will I still need to show a return flight ticket?

Don’t complicate your visa application. Stay under the radar with a cheap ticket out , and cancel it after arriving. GoAir gives fairly good refunds.

@K68

US citizen here. ... I’m not sure of how long I’ll be staying( I’ve planned on a year)

On the assumption you have a regular tourist visa, then you should only ever say that you are staying under 180 days.

Will I still need to show a return flight ticket?

That shouldn't be the case; particularly if you have a regular tourist visa and will be "staying 6 months".

What would be a good reason for having just a one way ticket

You plan to exit India overland to Nepal.

No need to buy return ticket.
If they ask -that is actually rare, youncan book expedia.com. not on expedia.in. dont buy any ticket from indian service provider (indigo, go air, and all those airlines). You will have time of your life to get the refund, and even if you get refund that would be partial. They are mostly fooling people.
Book from expedia.com and cancel within 24h, you will get ful amount.
Like others have said you can be there only 180 days continously.
You can get maried under special marriage act. Let me tell ypu it is a nightmare. Go to marriage department, they will publish both of your pics on the notice board fpr 30 days. If there is no problem then you can get married.
Also, in the registrar office the lawyer can charge usd 3000 his fee. Since you are foreigner.

Unless I’m mistaken the visas for US citizens means six months stay per calendar year. So this would involve a visa run.

Partially correct. As a foreigner you can't stay in India for more than 6 months in any given year. A visa run won't fix that problem. Not sure if marrying an Indian citizen makes a difference on that rule.

Not sure if marrying an Indian citizen makes a difference on that rule.

Once married the OP would be eligible for an entry visa; the tourist visa can be "converted" in country through the FRRO. My understanding is that the entry visa provides residence, but not the right to work.

After a qualifying period (2 years?) the OP would be able to apply for an OCI / PIO card (they've been merged, I don't recall which name is in use) - a lifetime visa with working rights.

I expect the OP is chapter and verse on the visa options after marriage.

There is lots of official advice online regarding getting married in India or even outside india and then continuing to live there, its complicated and full of red tape. My friend is about to do it after 9 years of 6 month stays in Goa and visa runs every 6 months.