This is an interesting question. I've actually been pondering this off and on for a few months. Of course any empirical answer to this would be incredibly complex and furthermore untenable because it's all speculation. However, I think a reasonable hypothesis is that the trends in sexual behavior we've seen over the past decades will continue in the direction they're going. Divorce rate had been escalating until just recently, but simultaneously a new trend of staying single longer has emerged.

I would venture to guess that economic conditions, women's equality (employment trends particularly), and improved efficiency in communication modalities (facebook, texts, instagram, email, and their progeny), all promote single status. If part of the motivation for some people to get married is related to economics, or scarcity of alternative potential mates, then the items above may decrease the benefits, and therefore the likelihood of people getting married. As the benefits of marriage continue to decline, fewer people are likely to get married, let alone stay married. Now imagine you're 30 years old, and you've been dating someone for two years and it's going well. Can you really say with any certainty that you know you're making the right decision for the next 230 years? So while people may still get married, I would look for the average number of spouses per person to increase dramatically among people who marry at least once. I believe this makes a future with alternative legal statuses to better reflect the future's culture more probable. Family Law (read: divorce law) will have to adapt to changing cultural realities so that two people can be allowed to enjoy the economic benefits of a marriage without risking huge losses in the event of a divorce. Right now we have pre-nuptial agreements that allow couples to define their risks beforehand, but this is an awkward proposition for those involved because it admits that faith in each other as partners has already been compromised on some level. The divorce process is so messy now especially when there is any complexity to the finances of the couple, that it deters the younger generation from saying their vows. We grew up watching men get crushed in divorce battles. It's not even to say that the results were unfair; it's just that the principal provider in a relationship stands to lose a lot of what they worked for (pensions, property, paychecks, etc.).

Another cultural shift we might see if we live to be 300 years old is people having kids at much later ages. People may wait until they are set-up well financially to have kids. If a 60-year old in the future has the body and energy of a 30-year old of the present, then it seems like it would be wise to wait to have children. Think about the lessons that we learn over that additional 30-year span; how much wiser we can be given 30 more years. And think about how much better the earning power of 60-year olds is compared to 30-year olds. I will make a disclaimer however: If in the future we don't really have "jobs" anymore, then the point I've made is moot.

TL;DR/Conclusion of it is that I think with longer lifespans it will be increasingly common for individuals to sleep with more partners.